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

@louisl 
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.

@lewm 
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!

@dwette 
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?
Thanks!