How come people don't make grammatical mistakes with...?

How come people hardly make grammatical mistakes with their native languages?

For exmaples, non-native speakers of English have hard time using proper articles(the, a, an) and prepositions(with, at, in, on, for...).
However, native English speakers do not make these kinds of mistakes and automatically use proper ones.

As another example, Korean and Japanese language have auxilary words and particles and I never mistake with them.
However, my Korean American cousin always get mixed up with them.

Why do we hardly make grammatical errors with our own languages but have hard time to use proper grammar forms with foreign languages?

I always thought this was kind of self-explanatory. Oh well, guess not every one has my insight into the crux of the human soul.

It's because you grow up speaking your native language. Duh. You don't make mistakes with something you've been doing your whole life.

It is because native speakers make the rules,

Native speakers with poor education make mistakes. For example one typical mistake in Spanish among poor and uneducated people is to say "me se ha caído" instead of "se me ha caído". Also it's very frequent to commit mistakes with prepositions and prepositional verbs in Spanish. For example, it¡s very spread the following mistake: "antes que anochezca". There is even a movie if I recall whose title is this wrong phrase.

"me se ha caído" who the heck says that? I've never heared it before.

Yes, indeed, even if I sound like a snob, while I accept mumbled speech of elder generations (the youngest being in their 40s, like my parents who always get some characters and very common words wrong), I can never learn the newspeak of my generation. This is Orwellian. Can they speak their native languages natively at all? Do they invent new small talks through mistakes? The situation is not as hard as the Shavian, but some sorts of so-called educated speech (i.e. by students, not school kids or those who are never students) are essentially atrocious. Then, I wonder whether they can think _at_ all. They do, but many language students don't deserve their identity for their utter lack of knowledge of the only language they can read without difficulty.