My guess is that evolution hasn't caught up with our big heads. Large brains and therefore large heads are a relatively new development. Big brains are highly selected for to the detriment of females. A big brain is worth a lot of pain, suffering, and even some death in childbirth. I'd imagine that the selective pressure would have over many tens of thousands of years selected for women who were more capable of passing the new human head without killing themselves (ability to have more children) or their baby. But, since we are so smart starting sometime around 4 or 5 thousand years or more ago we started to actively work against evolution with modern medicine. So we are sort of stuck in a transitional phase in evolution. Modern medicine has taken away the selective pressure that would have made child birth more comfortable as time went on.