You sound as if you would be shocked if someone were to say yes to that question. As you must be 13 to be on this site, I would think that your parents should have already gotten their citizenship by now. If not, why not.
We welcome anyone that wants to be a citizen of the United States, to apply to do so. What gets people in trouble is sneaking into our country, not obeying the laws, sucking up the benefits that should go to the legal citizens, etc.
You cannot deny that there are folks that are not citizens but come here to have their children so their kids will be citizens. I don't think it should be automatically granted, but that's my opinion.