Because no one ever brushed their teeth for them.
Ever heard of meth mouth? just saying...
My father-in-law makes (or at least made) meth. He avoided the embarrassment and discomfort of meth mouth by pulling out all of his own teeth with a pair of pliers. Yes, he's a democrat. Or at least, he would be, if he voted.
Pulling your own teeth out must be a primitive right of passage of sorts. At least my father-in-law didn't put his on a string and wear them as a necklace.