The more "options" one has the less likely they are going to compromise. We see this even in everyday marriages when two people have high powered jobs and make great money.
Imagine Angelina Jolie "kicking" Brad Pitt out of the house. There would be tens of millions of women around the globe who would welcome him in their arms. In fact many of these women would glady share him with Angie. Having that kind of fame,wealth, and options tests both your character and the love you have for someone; especially when your mate is not treating you well at the moment.
Another issue is many celebrities are away from home so much. It's hard to work on a relationship when you aren't there. They work on a project and then they travel to promote it while looking for the next project. Entertainers are for the most part independent contractors for hire. They're either working or looking for work. The famous also have to deal with stress that comes from the media, critics, fans, and stalkers. Some get to the point where they can't trust anyone. Diffrent people react to stress differently. There are those who escape by taking drugs, having affairs, or drinking. All of which could lead to divorce.