If he took say 25% of the shares, he could become one day soon the next Bill Gates/Elon Musk.
He's already worth more than a person can reasonably spend - why risk becoming Bill or Elon?
Let me point out the negatives of Elon
i) He is a bull-shitter. He has no clue about a lot of things, yet he has plenty of hubris.
ii) He is mis-trusting of people (this always comes from upbringing and environment. If people constantly misplace your trust growing up, your default is to mistrust people). I can understand why you wouldn't want to work under Elon. That's a perfectly valid reason
What makes Elon different, and which a lot of people don't get about him (including his fanbois)?
i) He is a true Engineer in the sense, even if he starts with bullshit and hubris, he is a quick learner. He breaks things to learn from first principles (Things he has repeatedly demonstrated everywhere). Media/HN focus on him breaking things, but that's what how everyone learns about complex systems.
People don't appreciate how quickly he went from from 'Alien Dreadnought' to 'Tent in a backyard' and then back to AI Automation at Tesla.
ii) Bias for action with the right amount of risks. Some people have bias for action, some people take risks and some are constant learners. But, I have never seen anyone combine all three like Elon. He is successful not because of his knowledge, but his willingness to take risks despite massive adversity and guaranteed mock by media.
iii) He is narcisstic, but not like Trump. He has several times shown self-deprecating humor and acknowledged his mistakes (and quickly learns from them).
But my dislike for both of them has nothing to do with politics or their wealth.
Musk is very much the same in his destruction of things that stand in the way of him and more money. His attitude isn't the ruthless businessman but rather the unrepentant troll - but his approaches to prevent the funding of things that stand in his way of Tesla ascendant (you get things like https://jalopnik.com/did-musk-propose-hyperloop-to-stop-cali... ).
Gates is largely over his quest for money and now more concerned with legacy, but that doesn't erase his history. Musk is still very much enmeshed in his quest for money.
I believe that Sam Altman doesn't want to have that be part of his goals and with the resources that he already has can fairly comfortably work on what he feels is meaningful now rather than waiting 30 years until he retires.
And perhaps that's exactly why he didn't take shares.