It's about power. It's just about power. Extreme wealth is a way for someone in democratic society to exercise the power of high office without actually having to convince the people to vote for you.
You hand me a few million? I'm never going to work again. I'll spend the rest of my days happily hanging with friends and family, pursuing my hobbies, and volunteering for causes I support. But then again, I'm not a psychopath bent on amassing as much power as possible.