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

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.

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.