There are probably big egos competing with each other. Not conducive to marriage whether or not you are a celebrity. The fact that one or both partners is probably better looking than the average citizen, so they get tons and tons of romantic attention from other people all of the time. There is way more temptation than would ever be put in the path of your average Mr. and Mrs. Joe Smith. Celebrities, as a group, tend to be more liberal minded and don’t necessarily see marriage as any type of sacred institution. It’s more of a publicity type of thing, and 2 hot people hooking up because they can, easily. Average citizens don’t have the opportunities to hook up quickly and repeatedly, in the same way that celebrities do. Then, there is the fact that if you are in show biz, you are likely to be extremely busy, and sometimes taken away from your home to work for extended periods of time, which often does not make the heart grow fonder, it often grows more distant.
So you got un-commonly pretty people hooking up quickly because they can, celebrity romance looks great on TV, so that’s another reason to get married, but because they did hook up quickly without considering the reasons that most real people get married, they don’t have a good foundation for marriage in the first place. All of this coupled with long bouts of separation and lots of other shallow pretty people set in their paths as temptations, make marriage unlikely to last.