When I was a student at Oregon State in the 1980s, I used to run up to Portland at the beginning of each term to see how many of the required textbooks I could find in the Multnomah County Library. For a few bucks in fines, I could save many dollars by not having to purchase vastly overpriced textbooks.
I did end up keeping one and paying the library what they asked for it. $15.50 bought me a $50 book, and I still have it (though I haven’t opened it in many years).
So your the reason math textbooks have a new edition every year. The math hasn’t changed but they make sure the only option is to buy a new copy.