This may be a weird thing to say, but….
I kinda think they’re both ‘too big’ to feel like they’re lucky to be in a relationship with anyone. Then. There is the familiarity from having been through each other previously. So, you wake up next to this person, and what’s your feeling? You can’t do anything to impress them. There’s no gratitude for being with them. And you constantly have other people fawning over you, appreciating/worshipping you. What’s the attraction to being locked into an old, unsatisfying circuit when you have more exciting options all around?
Then maybe there’s the issue of Ben’s alcohol issues? Maybe that’s still a thing? I dunno—I really don’t pay attention to celeb lifestyles, but these two just seem to have ‘eventual and obvious discontentment’ written all over them.
She desperately needs attention and he (who I think is a jerk) doesn't. Professionally he is much more stable as her career was basically over before American Idol put her back into the public eye. For all his annoying vibes his movie career has some solid items on its resume.