They aren't like thee and me down in LaLaLand. Larry McMurtry says that going to Hollywood is like going to a town of very powerful two-year-olds, and it's true. It's amazing sometimes that any good movies ever make it to the screen, the process is so arcane.
With an original book, it's the writer and the editor. In a tie-in, a shared universe, you have to add in the property owner.
With a movie, there are scores of people who can make it better or worse, and it only takes one or two to completely wreck it. You have producers, directors, actors, writers, film editors, composers, set designers, decorators, sound guys, and if all of them do their jobs well and the studio doesn't screw it up, maybe you get a great picture. It's a collaborative medium, and when it all goes well, the results are gold.
In Hollywood, a lot of people think the least important member of the above-the-line team is the writer.
And if the story isn't there, or it's badly-told, then you start out with two strikes against you. It's much more likely that a good script will get messed up than it is a bad script will get made better. You just can't make a silk purse out of a sow's ear.
