>>81516376Human birth is naturally incredibly painful regardless of what position you're in, your sacrum and sacro iliac joints are literally getting pulled apart as the baby's head is moving through the canal
With that said, being on your back with legs up is hardly the most comfortable position, it's only so popular because epidurals are so popular and you can't hold any other postion while under its effects, also the full labor for first time mothers can take many hours and often the last hours the first time mother is so exhausted she can't help but lie on her back to do the pushing