I don't think it's fair to say that the _whole_ textbook market is a scam to rip off students. At least nominally, the purpose of new editions of textbooks is to include information which has become relevant since the last edition was published.
I really only know one textbook author, Stuart Russell, who is writing a new edition of a textbook. He's taken a significant amount of time to revise old chapters and write entirely new chapters to include in his book, which is the most widely-used textbook on artificial intelligence. It's not just a renumbering of the end-of-chapter quiz questions.
In relatively new and diverse fields like AI, research results become standard practice fairly quickly, so textbooks have a legitimate need to be updated.
Term, holidays, term, holidays, till we leave school, and then work, work, work till we die. -- C.S. Lewis