Seriously, they do. Why? Well, there's not enough sequals. I know that's a weird complaint to have with movies, but it puzzles me.
You can have comics for any given super hero of group of heroes and have it go on for thousands of goddamn issues, yet when it comes time to make a movie, you get... what... two? Maybe three? Then, they either just stop making them or have some kind of "reboot" and start over. Why?
I say, as long as you can come up with good stories, keep making them. Have a dozen or more. If you can get so many comics written, surely a few more movies wouldn't be such a large hurdle.