Kewpie, first, welcome to the boards. I'm mostly a Survivor junkie, writing lots of parodies and summaries and songs and high-falutin' analysis and gunk, but lately I've been bridging out, kindof like the girls here. To answer your question: it's a homerun of reasons why ....
1. Ego
2. Narcissism
3. They're slumming for fun and thrills, since the jet-set scene's gotten a little boring lately.
4. Of course they don't need the money or fame, but MORE IS BETTER! Besides, this falls under the category of noblesse oblige.