Because babies are born with the capacity to learn but it takes a while to learn how to learn. 

More specifically, it's a causal relationship:

That’s the argument that Kidd and Piantadosi make in their new paper, published in the June issue of PNAS. Humans become so intelligent because human infants are so incredibly helpless, they argue; the one necessitates the other. The theory is startling, but it isn’t entirely new. Researchers have been pondering the peculiarities of our birth and its evolutionary significance for quite some time. Humans belong to the subset of mammals, called viviparous mammals, that give live birth to their young. This means that infants must grow to a mature enough state inside the body to be born, but they can’t be so big that they are unable to come out. This leads to a trade-off: the more intelligent an animal is, the larger its head generally is, but the birth canal imposes an upper limit on just how large that head can be before it gets stuck. The brain, therefore, must keep maturing, and the head must continue growing, long after birth. The more intelligent an animal will eventually be, the more relatively immature its brain is at birth.

Researchers have long known about this trade-off, and about the connection between brain size and neural density and intelligence. For instance, Robin Dunbar found that the ratio of neocortical volume to brain size can predict the social-group size in a number of species, including bats, cetaceans, and primates, while Simon Reader has demonstrated links in tool use and innovation to brain size in primates. Kidd and Piantadosi’s new idea is that increased helplessness in newborns mandates increased intelligence in parents—and that a runaway selection dynamic can account for both. Natural selection favors humans with large brains, because those humans tend to be smarter. This may create evolutionary incentives for babies that are born at an even earlier developmental stage, which require more intelligence to raise. This creates the dynamic: over time, helpless babies make parents more intelligent, which makes babies more helpless, which makes their parents more intelligent, and so on.

Panda babies have tiny brains, they grow a metric shit ton after birth. Just sayin'