Either way based on many CEOs track records healthy skepticism should be involved and majority of them find ways to profit on it at some point or another.
He talked recently about how he's been able to watch these huge leaps in human progress and what a privilege that is. I believe that - don't you think it would be insane and amazing to get to see everything OpenAI is doing from the inside? If you already have so much money that the incremental value of the next dollar you earn is effectively zero, is it unreasonable to think that a seat at the table in one of the most important endeavors in the history of our species is worth more than any amount of money you could earn?
And then on top of that, even if you take a cynical view of things, he's put himself in a position where he can see at least months ahead of where basically all of technology is going to go. You don't actually have to be a shareholder to derive an enormous amount of value from that. Less cynically, it puts you in a position to steer the world toward what you feel is best.