Bill wrote:
Why are bath/shower valves mounted so low?

Perhaps because this was first done with clawfoot bathtubs (no wall to
mount to) and they have done it that way ever since?

Seems to me you are always turning the water on/off from a standing
position for either a bath or a shower.

Not necessarily; maybe you want some more hot water before you are done, or
perhaps you might get in the tub before it is completely full. There's
also children who may want a shower, and they won't have to reach as high.

In my recent bathroom redo I left the handle where it was (32" from tub
bottom), mainly out of laziness, but I did mount the shower nipple up
higher, pushing it to 82 from the bottom of the tub.

Jon