According to this article, AMD’s senior manager of developer relations says that game developers only use PhysX for the cash. Thanks to Edward Cabarles for the link.
"They’re not doing it because they want it; they’re doing it because they’re paid to do it. So we have a rather artificial situation at the moment where you see PhysX in games, but it isn’t because the game developer wants it in there." In fact, Huddy reckons that no developers outside Epic genuinely wanted to implement GPU-accelerated PhysX in their game.