It’s the grammar. We cannot say “much late”, because we don’t use “much” to modify an adjective. When we say “much later”, the word “much” is not modifying the adjective, but only the adverb “more” (In this case, as the suffix -er). To strengthen “late”, we could use another adverb, such as “very” or “really” late.
Very intelligent. Much more intelligent.
Very smart. Much smarter.
Let me know if this explanation was clear and helped you.