Largely, because books are heavy, and there's limited demand for most used books. If you've ever been at a book sale on the last day, there are always lots of leftover ones that no one wants.
There's a general sense that librarians are preservationists. This is far from the truth. Most books get pulped within a few years of printing. Librarians are constantly discarding books that don't get checked out to make space for new books.
In the name of efficiency they cut the spines, rip the pages apart for easier scanning, books are literal dead wood which would cost a lot in shipping so it's easier and cheaper to just discard or burn the remnants.
The book ceases to be a book right at the beginning of the extractive process.
