Why don't all phono preamps include a rumble filter?

In my system, the use of a rumble filter is mandatory (I notice no degradation of sonics when I engage the rumble filter).  I'm thinking of upgrading my phono preamp and would like to choose one that has this as an optional feature.  However, it appears that most manufacturers of phono preamps do not include this.  Why not?  Does the use of a rumble filter degrade the sound of a phono preamp?
In a modern, properly specified, and correctly installed, aligned, and calibrated LP playback system no rumble filter is necessary because there is no rumble of significant presence what are you using for your LP playback system if you tell us we may be able to help that is where your problem is.
 There are many purist  types who would reject the notion of using a rumble filter for just the reason that you suggest in passing. Which is to say the filter can degrade sound quality. After all, the simplest filter you can construct would be a passive one that would use a capacitor there is no such thing as a totally transparent capacitor. But also you would need some active components, so that you do not lose gain as the signal passes through the filter.  All of that said, I think KAB make a rumble filter that can be used outboard of any preamp. Check their website. What I myself would do is to find out why my system needs a rumble filter in the first place. Once I have figured that out, I would correct the problem, rather than adding a filter. 
Thank s for these quick responses to my question.
First:  @clearthink 
my analog system is a VPI Classic 3 TT, using either a Lyra Kleos Cartridge 0.5 mV, or a Spectral Reference Cartridge 0.2 mV. Using High Fidelity Reveal interconnects, into a Acoustech (Sutherland) phono preamp.  The TT is wall mounted, on the wall behind the speakers (Legacy Focus SE's), augmentedwith a pair of REL Storm III subs).

Second:  @lewm 
i am using a KAB rumble filter (with great success).  It is wired in to my Tape loop of my preamp (Audible Illusions L3).  I notice absolutely no degradation of sound when it is engaged, only the removal of an rumble.  My room is 14 1/2' x 19'.  At low volumes, I don't need the rumble filter, but at any 'realistic' volume (90db or above), I need it. The way my room is set-up, there is no way to move the TT to a different location in the room.
Nakamichi also used to make an inline filter that I believe was down around 10 that worked very well, with that said with a table such as yours there should be no need and only take away from it all.
I had 3 different  VPI turntables, a HW19 MK4, Scout And Prime.  All 3 of them neccistated a KAB Rumble filter.  Like yourself, I couldn’t move the turntable from its position. I tried everything I could think of, 4 different types of platforms, different kind of footers, etc. Nothing changed the rumble.  Recently I switched to  Technics SL1200G turntable, using the same cartridge and lo and behold, no more rumble.  It must be something in the design of the VPI tables that helps amplify the rumble.  At any rate, I am very pleased now.  
I was Just thinking, as you and I are kind of in the same situation, one of being shoe horned into a given set up in a less than favorable room, you might want to try a Symposium ISO platform between your table and shelf in your wall rack. 
interesting, that you've owned 3 different VPI tt's, and you had the same problem with rumble, until you inserted a different TT.  Hummm?  Makes me think.

i have a 3" maple platform, mounted on top of multiple 2" square rubber/cork/rubber isolators, on top of my wall mount.  However, I agree with you that, "with a table such as yours there should be no need".  Unfortunately, the need is present.

To restate my original question, why wouldn't the manufacturer of a high end phono preamp include an option to include a rumble filter.  If it's not needed, the user could disengage the filter, if it's needed (and obviously there are many others who could use this option) the user could choose to engage it.
I believe that in the truest sense they want there product to shine in its best light, kind of forcing us into addressing any issues and not making any compromises. Just sayin.
There is no reason to think about changing tables, I too have had three VPI tables never having a rumble issue with any of them, all supported by a lead balloon turntable stand so I started out with a good set up from the ongo. Symposium makes an insert to replace the rubber grommets that vpI uses in their feet that really work wonders for around $200.00. You should be able to get that rumble thing under control with very little effort.
Because 1) regardless if the filter was engaged or not, it’s another switch in the signal path, 2) if you’re gong to implement it with any transparency in mind, it’s going to require a high quality capacitor (expensive), adding cost to the product, and 3) may require a change (more complexity) the the circuit design. 
Have you inspected and lubricated the main bearing lately?  My HW-19 MK IV had no audible rumble at any volume.  The Technics in stock form may be more immune to feedback.  Could it be that you are referring to acoustic feedback as rumble?  As noted, there are solutions for that.
My personal experience is that a rumble filter is a band-aide (and a poor one at that) for not correcting isolation/vibration issues. I learned that the hard way and now have a KAB RF-1 Rumble Filter sitting unused (and unneeded) in my closet.

Agreed, but sometimes it is the ONLY option after all else fails.
@stereo5 In the end, my primary problem – after I got isolation under control otherwise – was the tonearm. I was using a Clearaudio Magnify. The magnetic bearing caused severe woofer pumping on certain recordings. After doing everything else I could to isolate the problem I finally changed to a tonearm with mechanical (precision ball) bearings. That was the night and day difference that finally solved my rumble issues. I also had to correct some isolation issues, and now I can play bass heavy records at high volume (and that’s with Confidence C2s and a pair of RELs) with no rumble issues at all (unless the record itself is mastered badly in that regard...rare for my collection).
infrasonic rumble can destroy you woofers. especially when listenning at high volumes! My NAD phono preamp does not need a rumble filter as it always cuts those ultrasonic noises. With my former phono preamp (ATOLL) woofers were moving excessivly producing infrasonic bass. 
its been maybe a year since I have lubricated the main bearing, but when I did, I didn't notice any change.  However, thank you for reminding me that it's time to do that again.
Also, isn't acoustic feedback the same as rumble.  If not, what's the difference?
a rumble filter may be a band aide, however, when engaged, I notice no loss of anything (transparency, detail, staging, etc) other than the pumping of my woofers.
ok, so in the purist form, inserting a filter in the circuit may not be 100% kosher, however, if your traveling down that avenue, why do designers use crossovers in their speakers rather then design them cross-overless, why use a speaker cable with connectors on each end (rather then use bare wire, and solder it directly to the amp/speaker), why do designers of electronics use IEC connectors for the power cable, rather then hard wiring it (as some do) on and on.  These all affect 'ultimate' transparency, and are design choices.  I just don't believe that any of the things you mention can't be done with no loss of performance, or ultimate cost (1-another switch in this signal path....how many switches/connections  are already in the signal path, are you saying one more switch is going to degrade the sonics to the point where it's so audible that it is going to degrade the sound of your system to a point of dissatisfaction?      2-, after all, in a phono preamp costing several or more thousand dollars, how much more can a 'high-quality capicator' add to the final cost of a product? 3-a change in the circuit design...ok...I agree with that.  I am not trying to be combative with what you are saying, I just believe that these items that you mention can be dealt with in an effective manner.  Happy New Year!

There are just too many factors that create rumble or acoustic feedback (still don't understand the difference), and there are too many people that experience this problem.  I use my KAB rumble filter, with no loss of sonics, so I want a phono preamp with this included.  The only negative I have with the KAB is that another set of interconnects is needed.
I guess it depends on the system, but when I engaged my KAB RF-1 – whether in a tape loop or inline between phono and pre – I experienced a loss of SQ. That is why I made the effort to fix the root cause of the problem, rather than patch it with a kludge.
Louis, with whom are you arguing? None of us is against the idea. Many older phono stage designs, especially from japan, DID incorporate a rumble filter. So if you really want that feature, find one of those.  American products from around the 1970s maybe up to the 1990s also can have rumble filters built-in. The lack of a rumble filter on modern phono stages is partly a symptom of laziness among manufacturers, and their desire to cut costs while also raising prices. And finally, the outboard KAB filter is really no different from what you would get if it were built into a phono stage. So, you can add that function to any phono stage, via the KAB filter which you apparently already own. But first of all, I think it is important for you to figure out the source of the rumble. Rumble is just another word for low frequency noises. There are many causes. Sometimes the problem can be cured without resorting to a filter. For example, is your cartridge well matched to your tonearm? Perhaps the resonant frequency of the tonearm cartridge combination is too low, causing the system to resonate due to foot falls or subtle record warps.
By the way, if you don’t like the idea of an extra pair of ICs in order to incorporate the KAB filter, you could hard solder the device in-line with your preferred phono ICs. That would satisfy even a purit zealot.
sorry, don't mean to sound as if I'm arguing.  It's a New Year, and I have too much to be thankful for!
My TT is very well isolated.  I can literally do jumping jacks directly in front of my TT, without affecting the tonearm while it is playing a record.  So, no problem with foot falls.  I believe the problem is acoustic feedback (volume low....... no rumble, turn the volume up....rumble).
You bring up a great point about the matching of my Cartridge and tonearm.  I will check on this!  That may be the problem.  Thanks for that suggestion!
Bottom line, like you,  I would like to identify the source of the rumble, and correct it, rather then use the KAB filter (however, I really hear no drawbacks when using it).
Hope your having a Happy New Year, and again thanks for your input!
Happy New Year!
when I engage my KAB (in my tape loop), I hear absolutely no degradation in sonics, so, as you say, I guess it is system dependent.  So, what was the root cause of your problem with rumble, and how did you correct it?
"Rumble" is a misnomer since low frequency noise can also be caused by pinch warps or a wavey surface on an LP.  Such defects are exacerbated by a non-ideal tonearm mass/cartridge compliance mismatch.  You can visually see this sort of problem by looking at the woofer cones flutter while a record is playing.  This is a good reason for such a filter which typically engages below 20Hz.
Other than getting my turntable properly isolated: I use a Solidsteel WS-5 wall bracket with 3" end-grain maple block. The turntable (Clearaudio Ovation) sits on that. I have changed the OEM feet to a set of Harmonic Resolution Systems (HRS) Nimbus pucks (I have also tried isoAcoustics Gaia and Orea feet...they all work).

As explained in my post above, the real source of rumble was the Clearaudio Magnify tonearm I had...the magnetic bearings were the issue. I changed to a Clearaudio Universal arm with mechanical bearings. Problem solved completely. My KAB sits in a drawer. It isn’t necessary. Fixing the root cause is still the best way to solve problems.

Likewise, Happy New Year to you and everyone else.

I believe the “rumble” you are experiencing is due to sound waves acting on the wall, where it is a moving diaphragm, transmitting the low frequency impulses back to the turntable, which has inadequate isolation. If you can move the speakers outside your listening room as a test, try listening to the same LP on the turntable on the same wall mount location. I believe you will find the “rumble” absent. 
You said "Also, isn’t acoustic feedback the same as rumble. If not, what’s the difference?"
I’ve always thought of rumble as something audible or inaudible (pumping) with the volume turned up, the table spinning with the needle down and no music playing, and acoustic feedback as an external excitation by the speakers.
Some older Japanese integrated amplifiers used to have rumble filter switch.  I had a Kenwood amplifier from 1978 that had a switch labelled "Subsonic" and the manual specified -10dB/octave starting at 20Hz.  The problem with all analog component filters is that they introduce frequency dependent phase shift and the sharper the cutoff the greater the phase shift. Rumble used to be a problem with old 78 rpm hard shellac records played on older turntables with relatively imprecise bearings. Modern turntables playing 33 rpm LPs made of PVC have rumble specifications that are trivial.
Rumble is mechanical (audible and inaudible) noise transmitted to the LP due to inadequate decoupling between the LP and the platter and drive mechanism. Rumble can be mitigated by the combination of well-balanced motor (electrically and mechanically), well-designed and maintained platter spindle bearing, sufficiently damped platter structure and sufficiently resilient platter mat.