If you want to understand millionaires who continue to do startups, here are a few possible reasons:

1) They enjoy work, even if they are working on something that is seemingly trivial. 2) One million dollars is not enough. Nor is 10. In fact, most people that earn a few million rarely retire after that. It is more a game of power, fame, and (possibly) goodwill than wealth. Bill Gates could have retired much earlier, but instead he amassed a huge amount of wealth and started one of the world's biggest charities (the Gates Foundation). Warren Buffet spends very little on himself - he owns one house that he bought for a small sum of money, and he gave 99 percent of his money to the Gates Foundation.

That said, there is a fine line between the "punk kids" (who are usually the designers and developers) and the founders in larger companies. Founders in their second, third, and etc...companies typically have very comfortable lifestyles.