They are uneducated, and most do not care to learn. I can tell so much about someone's level of education by their word choices. They're, their and there are my biggest pet peeves, but I have too many to list. Poor grammar is a reflection on a person, but the people who are the worst are not even aware they can't write well.
It has a lot to do with when children are growing up, whether they are reading many books, whether they have a good education, and whether they have good role models in that aspect of life. If you grow up with illiterate parents, siblings, and friends it is far more likely than not that you will grow up illiterate.