A baby is born with a brain that still needs to change itself. It is not yet ready to make complex decisions and to fully control the body. It will need to establish the rules on how it decides and how it will operate the body. Most of these changes will happen in the early childhood years, though the brain will continue to change even as the baby becomes an adult. It is just that the changes that an adult brain can make are much smaller than a baby's brain.
Most importantly, the ways of learning that the brain uses as a baby will be the same rules it will use as an adult. That's why caregivers need to show the baby good ways to learn!