I think I've read that he just decided to focus on other things outside of filmmaking, he's certainly had enough success that he probably doesn't need to unless he really wants to. Whereas someone like Donnie Yen (same age) whose been working for a long time, but didn't get as many major roles until more recently probably still wants to show what he can do. And of course other older action stars each make different choices about what kinds of stuff they want to do later in life.