I can't decide what my theory is. I mean, I don't recall choosing to pay attention in elementary school, but I must have, and passable spelling/grammar is a skill I totally take for granted now. It blows my mind how some people just don't seem to have absorbed it.
Did they learn it in school, and then stop caring? Why would you stop caring if you know that poor writing skills make you look lazy or unintelligent? Did they not pay attention in school, or did their teachers not care, and just pass them anyway? I just don't get it. It bothers me every day.
Bonus questions: Do you know of any published research that has been done on this phenomenon? Is it really a phenomenon, or do you think it's always been the case, and wide access to the Internet just makes it more likely for more people to be exposed to bad writing/spelling/grammar/blah?