Publishing deals often revolve around exclusive options. Also, publishers often wants to bring in a third person as an editor, market a book or a film at a particular time during the annual release cycle and so on - often for entirely legitimate reasons. It's not that publishers are inherently morally worse tha autheros, just that their interests aren't perfectly aligned, and short copyright terms shift the incentives strongly in favor of the publishers because they have more economic resources than the typical author and can better afford to play a waiting game.