Think about it. Hollywood marriages usually just equal messy divorces -- but it's not like Angie is going to sue Brad for spousal support. She does pretty well on her own.
As for the four kids they have (and Angelina is pregnant again as we speak) they all bear the name Jolie-Pitt, so they have the gravy train coming in from both sides. Why mess with a good thing?
Bottom line: they're better off just doing the Kurt and Goldie thing.