The rationalization most moviegoers would use is one of legitimacy. Many people see film as the only "serious" medium for storytelling (live action TV is too cheap, books are too academic) and categorically dismiss newer media (video games, comics, etc.) as having no cultural value whatsoever. Thus, a good story is not actually good until it's a movie.
Hollywood loves this attitude and may be entirely to blame for it. It's hard to tell how many studio execs actually think this way though. I get the impression there are also a lot of people in Hollywood who don't believe in the concept of art.