Brains don't have innate grammar more than languages are selected to fit baby brains. Chomsky got it backwards, languages co-evolved with human brains to fit our capacities and needs. If a language is not useful or can't be learned by children, it does not expand, it just disappears.
It's like wondering how well your shoes fit your feet, forgetting that shoes are made and chosen to fit your feet in the first place.
It's not an either/or. The fact any human language is learnable by any human and not by, say, chimpanzees needs explaining.
Chomsky also talks about these kind of things in detail in Hauser, Chomsky and Fitch (2002) where they describe them as "third factors" in language acquisition.
You could say that languages developed ("evolved") to fit the indisputable human biological faculty for language.