Electricity was not in widespread use when these machines were built.
There were attempts to do the very thing you suggested, but pedal pumpers
outsold them a thousand times over.

The piano companies needed a player that would operate anywhere to insure
sales.  Most rural areas didn't have electricity and a electric piano
would have been useless.

Nowadays there are solenoid operated players:  Pianomation, PianoDisc
and others.  But they can't play unless they are plugged in, whereas an
old pedal pumper will play out in the cotton field if you want it to!

Andy Taylor
Sikeston Missouri