Most short keyboard have a "octave shift" function that moves the keys left or right so you can have it any way you like.

dewster the wurlitzers start on A, casio normally starts their 76 keyboards on C, but the biggest influence was the rhodes electric piano built by harold rhodes for injured soldiers in world war 2 as a part of their rehabilitation.

The first models that harold built sounded much better in the lower registers than the upper ones, so he started it at E so it could replace the upright bass. Later, when Fender got Rhodes to build a 32 key version that only has bass, it also started on E. Fender didn't like the sound of the upper register.

But by the mid 60's harold had fixed the electric piano so it sounded good in the full range of a regular piano all 88 keys, but due to history, the shorter 76 electric pianos still started on that E, to this day.

So there you go. It's some combination of history, and matching that bottom E of an upright acoustic bass, and the demands of Leo Fender.

but not all digital keyboards start on that E, some start on A, others on C.

Lowest note being E, is most useful for playing left hand bass, in lieu of the absence of a bassist.

My wife claims she uses the notes below that E all the time. Or often enough to really miss them when they're gone, anyway.

right now i'm used to having a C as my bottom note, I have one 5 octave and one 6 octave keyboard both of them start & end on C.

It really sounds like your wife would like an 88 key keyboard more.