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

"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.

@louisl
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.

@louisl 

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. 

louisl,
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.