It's easy to get your 15 minutes. But your 15 minutes does not always equate wealth. Which means these people are famous for a little bit, but they aren't going to make that much money off of it.
And I think it's easy to lose that fame. There's always someone new doing something new, so what was once famous, is now so last month. These people don't have the staying power of someone truly famous.
The truly famous are not famous for a single novel event, they are famous for a series of performances or products. Not because they danced cute when they were 16.