If they are going to spend a ton of money to make a movie, they require some assurance they will get their money back.

And sadly, about the only surefire way to do that is to make movies out of established franchises. Making a movie that isn't a sequel or remake of another successful movie/TV show, isn't based on a bestselling book, or isn't based on some other form of pop-culture icon that already has traction in the marketplace (e.g. superhero movies, fairy tale movies, Transformers, World of Warcraft, etc) is very risky.

The people who back big-dollar movies don't like risk.