Because authors have not even the first clue about how copyright works. This is at least partially by design, because if they did, nobody would sign ownership away to a publisher. The reason why publishers want ownership is specifically because they get paid way more in the derivative works market than by selling copies of a book.
Derivatives are also the reason why the plaintiff lost. Fun fact: fan fiction is not copyrightable, so if you make a derivative work without permission you own nothing. Yes this means it's legal for the original author to just resell your fan work as their own, as the plaintiff alleges (even if it didn't actually happen here).
Derivatives are also the reason why the plaintiff lost. Fun fact: fan fiction is not copyrightable, so if you make a derivative work without permission you own nothing. Yes this means it's legal for the original author to just resell your fan work as their own, as the plaintiff alleges (even if it didn't actually happen here).