If a publisher pays a writer some money to license the writer's copyright, then the publisher can print and sell copies of the books. But if a different publisher can do a better job, customers might buy that other publisher's nicer books, and the writer might not get any money. Or, more likely, another publisher makes the books cheaper and everyone buys those. The customers are still getting the same text, maybe an exact duplicate of the licensed copies, but without copyright the original author might not get any money at all.