If babies acquire grammar naturally, why do you keep harping on arguments and evidence in class assignments?
Good question--as I said, we're trying to make explicit some of the rules that native speakers of English acquire naturally, but demonstrating how those rules work is not a natural process. It's very much a matter of presenting arguments. Grammar (or linguistics) is not a field of givens to be memorized, but a field that studies linguistic structures and deduces and argues for rules governing those structures.