Why can't a fair number of people grasp the difference between "there," "their," and "they're"?

I see them misused a lot. It ain't rocket science.

This was taught to me around the 6th grade at the latest [1975]. Do some people really not pay attention in elementary school?
Update: The first answerer [a brilliant troll] reminds me: The same goes for "your" and "you're," which are misused even more!
Update 2: FYI, my use of "ain't" was intentional and for comedic effect. Well, SLIGHTLY comedic, granted . . .
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