I have no idea, but i suspect a lack of knowledge and the fact that chairs probably already existed when outhouses and such first started getting used, so it was probably easy to just copy that design for a place to sit down and poop. And nowadays it's too much "work" to change an already established and iconic design.
>>95498Yeah, this is the way to solve it.
Incidentally, wearing boots with tall platforms can also help getting you in a more squatting position while on the toilet.