What other reason could there be?
Wealthy folks aren't necessarily content and are always on the lookout to make even more money. The desire to acquire more, I would say, is in all of us. How we go about curtailing the urge/desires is left to the individual. I think it's only natural to want to be better and grow, thus the crave to want to acquire more.
If you have a million bucks today, you'll want to have two million the next day and ideas on how to grow your money would be running through your mind. It's no competition and nothing personal, it's just business
Also, a wealthy individual can chose to adopt any lifestyle he/she wants. A lot of people would love to have that option, hence the constant drive to increase ones net worth.