Consider that a body doesn't know how many limbs it will have, or what they will be like.
"How to move your limbs" isn't something that can be hardcoded, but "How to learn how to move your limbs" is.
It is the same with the senses (you might be myopic from birth, or hard-of-hearing or something else, or you might be that lucky first mutant with a functioning third eye at the nape of your neck) - so the brain can't have how-to-use hardcoded, but can have how-to-learn-how.