Why do doctors have moms give birth on their back if it closes the pelvic outlet up to 20-30%?

Childbirth & gravity: In Native American other cultures childbirth is accomplished in a squatting position. In "modern medicine" childbirth has been done in a dorsal lithotomy position(back). Why? Probably due to access for the Attendant to facilitate the delivery and provide emergency care. With the continuing rise in C/sections "back position delivery" may soon apply only to C/sections.(smile)