After all, they've been speaking it since babyhood. They are surely intimate with it by now. What this does not take into account is the many differences between a spoken and a written language. You have no need, for example, to understand spelling and the rules of punctuation in order to accomplish oral communication. Perhaps you can see from this alone that intimacy with a spoken language is not sufficient for the writer.
Punctuation presents some of the same problems.
In speech you know when to hesitate for a comma and when to come to a full stop for a period. You've no need to know when spoken words should contain an apostrophe and when not. The person to whom you're speaking will determine your meaning from context. If, however, you, don't. I am dismayed to have to tell you that a great many hopeful writers do not.