His stock was worth $320 million after the IPO, and he let it all ride. On a single stock in a two-year-old company. Nobody could afford to do that unless losing most of $320 million wouldn't affect their lifestyle. Being wealthy explains most of it, but I suspect being 72 years old was a factor, too.

Some jobs are not just about getting enough money to retire, they can also be about making the world a better place (or sometimes just believing that's the case), just living a passion, just having a social activity and many other reasons.