Biologically we carry the child, we nurture the child after they have been born. Our hormones and the baby's hormones complement each other beautifully in the hours and days following birth to facilitate this relationship and encourage bonding, because it is so important. As PP said, our children are connected to us physically at a young age, this is not because we are brainwashed by society, its because we are needed to feed and nurture them. And again, this doesn't diminish the father's role, but he plays a different role.
I do agree men need to step up and be more involved in parenting, but as of yet, they largely haven't, and this is why I think mothers are so important.