The other kinda dumb constraint is that our heads are roughly as large as they can be and still survive vaginal delivery.
Slippery metric. "Requiring" seems hard to isolate vs other options like "elected to use".
Supposedly more common as a choice amongst higher income women. Hence the phrase "Too posh to push"
I admittedly havent spent much time thinking about the problem, but it seems to me that the issue of getting one human out of another human is quite a difficult problem to solve.