Things like strokes or cumulative CNS damage primarily affect older people, so genetic fixes to those problems would have very little evolutionary selection pre

Things like strokes or cumulative CNS damage primarily affect older people, so genetic fixes to those problems would have very little evolutionary selection pressure on their side. It's also possible that genetic/biological means of turning on CNS regeneration in higher mammals would require a complexity that has not had time to evolve yet. Just because we evolved to the point of human-level intelligence doesn't mean we are perfectly evolved by any stretch of the imagination. It means we are, in a sense, a minimum viable biological entity that can acquire our level of intelligence.