This is a better way of putting your point.
You have a point. It all depends upon how you learn grammar, and that normally begins in the home. If the parents speak good, correct English then the child should grow up knowing intuitively what is right and wrong. But the longer a child is exposed to incorrect English the harder it becomes for the child to choose the correct grammar - that is when teaching the rules becomes essential.
An example of this is the double negative. "I don't know nothing", which should, of course, be either " I do not know anything" or "I know nothing". There are areas where this usage is endemic and no amount of teaching has managed to erase it, probably because the child may well understand the teacher whilst in school but surrounded by everyone else outside school using the double negative will fall back into that regime.
Very few of us, including regrettably myself, always get all our grammar correct. It is, therefore, helpful to be able to look up correct use when in doubt, so having the rules set out must be a good thing.