Of all the answers Resident Alien could have given us with regards to why the Greys abduct human babies, we did not expect the revelation provided in Season 4, Episode 6, "Soul Providers." Following ...

Human newborns arrive remarkably underdeveloped. The reason lies in a deep evolutionary trade-off between big brains, bipedalism and the limits of motherhood.

A newborn baby cannot walk, feed itself or even lift its head. Compared with most animals, human babies are remarkably helpless. Yet this dependence is not a weakness; it is an evolutionary advantage.