Because he values something else more than getting rich. E.g., achievement. Or even getting filthy rich.

Note that Mark Zuckerberg could have sold Facebook early on for a variety of "he gets rich" prices: http://www.businessinsider.com/all-the-companies-that-ever-t...

And I'd add that anybody who gets into doing a startup only because they want to be rich is probably better off buying lottery tickets.