I think he does these movies because not only do they not take very long to film (The Purge took only 20 days), they make him money so he can focus on his real passion, writing, directing, and doing small films like the Before Midnight. He at first was hesitant to do horror as it apparently took a lot of convincing to get him to agree to do Daybreakers. That film wasn't much of a hit either but he obviously liked the experience enough that he's doing Predestination with the same directors.