He's a movie star who doesn't make any good movies. Oh he has been good in a couple of Coen brothers flicks. And I really liked Out of Sight, but that was based on an Elmore Leonard book and directed by Stephen Soderbergh. Hard to screw that up.
Monuments Men, Leatherheads, Confessions of a Dangerous Mind: none of those was going to break any box office records.
Clooney brought in money with the Oceans movies, so Hollywood lets him do what he wants so long as he doesn't actually lose a lot of money.