How does a baby deer stand the day it's born?
If you compare placental mammals in how much time they need to start walking, you'll see that deer are no exception. Humans are an exception.

Hypothesis of Obstetrical Dilemma

The hypothesis of Obstetrical Dilemma states that humans are born premature. We very much think this is because if we were to be born more developed (like other mammals), our big brain would not be able to make its way through the pelvis. Also, bipedalism leads to a narrower pelvis making the passage of the big brain even more problematic. For this reason, human babies are very dependent on the care of their parents for a long time. This hypothesis is called the Obstetrical dilemma (see Rosenberg 1992, Weiner et al. 2008 among many other papers as well as several books such as Ancient Bodies, Modern Lives for example).

Counter arguments to the Hypothesis of Obstetrical Dilemma

Note however that this hypothesis comes with a few potential contradictions, such as the fact that human gestation is no shorter than the gestation of humans' sister species. Indeed, in chimpanzees, for example, gestation lasts 243 days on average against 280 days on average for humans. These counter-arguments can be found in Dunsworth et al. 2012.



Thanks to @MattThrower and @AdamDavis for their helpful comments.