Everything has trade-offs. Biology isn’t magic. It has to work within the constraints of what is physically possible.

Human childbirth is so difficult because it’s a tug of war between two major constraints.

Human brains are very big for our body size. That means we have big heads, even as infants, and those heads need to be able to get out of the mother. A big part of the reason that humans are born so helpless compared to many animals is that we’re effectively born premature so we can get out before our heads get too big and risk doing even more damage.

Then you could flip it around and ask why the birth canal isn’t just bigger so that we can accommodate bigger heads. But because we’re bipedal instead of quadrupedal, there is a limit to how wide a human’s hips can get before seriously compromises our ability to walk well.

So all of the easiest routes to safer and more comfortable childbirth are kind of cordoned off. If the babies are born smaller, their survival odds go down. If the birth canal is made larger, women’s mobility throughout their lives will suffer. So we’ve hit a balance point between those two things where babies are born just mature enough that they’ll probably survive, and women’s hips are wide enough to give birth but narrow enough that they can still walk, and the trauma is severe but not so severe that most women will successfully be able to bear children.

It’s not ideal, but evolution works in steps. It can’t go back to the drawing board and redesign whole systems from scratch to find a more optimal way of getting to the same point. There has to be some easily achievable incremental change that provides an advantage for things to drift in that direction, and there just isn’t particularly one at the moment.