First of all, I saw Michael Clayton and it was a great film.
The poster for the film shows a picture of George Clooney and the film is named after his character. I personally don't believe this is a film centered around one person. Clooney might be the person with the most lines and screentime, but I saw it more as a film about a relationship between Clooney and Wilkinson. There relationship reminded me a little bit of Rain Man in that respect: a sane guy having to deal with a brilliant insane guy. Tilda Swenson and Sydney Pollack also play significant roles.
Lions for Lambs, for example, markets itself as "Cruise, Streep, Redford" and supposedly Tom Cruise is an egomaniac. Why couldn't this film have been "Clooney, Wilkinson, Pollack"?
I think the answer is that Clooney is just a humongous star right now (I personally have no problem with George Clike him, I think his star is just shining a bit too bright, in my opinion) and the best way to sell the film is to market it as a George Clooney vehicle.