True, but imagine the hassle of having to make sure all PI was protected, all relevant mechanics were declared open, and all mechanics utilizing PI terms and concepts were either reworked into open versions or locked down? All with clear statements to tell publishers what's what?
Not so easy.
Besides, I'd never expect a movie license game to be built upon or to support open-source material. It's kind of like releasing a major motion picture and then making many parts of the movie into public-use stock footage.