Why the development of linguistic skills involves the acquisition of grammatical rules rather than the mere memorization of words and sentences?

Why the development of linguistic skills involves the acquisition of grammatical rules rather than the mere memorization of words and sentences?

here are reasons for believing that the development of linguistic skills involves the acquisition of grammatical rules rather than the mere memorization of words and sentences. In principle, no human brain can store all the words and expressions of a language. What happens is that when processing the language they hear, children construct the grammar and make sense of the expressions according to the grammar. When producing utterances, they follow the internalized grammatical rules, such as the X-bar rules, that allow them to associate sound and meaning for a potentially infinite set of expressions. Without the knowledge of the productive rules, it would be impossible for language users to produce and understand an unlimited number of novel sentences.