> When they're damaged beyond hope of repair from decades of wear. As for books that haven't fallen apart from being used as intended and merely are no longer desired, my local library generally sells them off at bargain prices. Of course I can't speak to your local library.

Many books aren't lent and not bought and most libraries have limited space to store such books, thus they go where old paper goes.

Of course some rarely lent books are important and for the one person asking for it in ten years really valuable, but many still have to go.