They don't wear out, but colleges usually use a given textbook for only a couple of years. I myself am absolutely certain that there's quite a bit of payoff from the publishers to the state buyers that makes it necessary to get a "new edition" of a calculus textbook every five years or so.
But in college, the student buys his own books. He can choose to buy used and get a huge discount, or buy new and eat the mammoth price tag.
In public high school and down, the school district does the buying, and only when it’s strictly needed OR when the state board decides on a new edition.