Gabe was already wealthy with Microsoft money when he founded Valve, so when his new private startup found success he didn't feel the pressure to go public, expand, dilute, and cash out. He made the judgement call that they already had enough talent internally to keep playing the hits while keeping all the profit for themselves, and he was right. I'm sure a little bit a business ideology reinforces Gabe's long-term outlook for Valve, but he's ultimately enabled by a happy intersection of pre-existing wealth, great timing, and careful hiring choices.