He wanted to be the CEO and merge it with Tesla[0], but the researchers had a problem with him (some had a problem with Altman as well, but that's another story). He did not have any real options since OpenAI was a non-profit then, so he just left. The new book The Optimist[1] about Sam Altman has some more details on this and other OpenAI Game of Thrones, I definitely recommend for those interested.
There's also the small detail where OpenAI decided to only remain open in name?
And the fact that Sam from the very start wanted to turn it into his own closed source for-profit company (still ongoing) using non-profit funding as start-up seed funds (essentially stealing Elon Musk's money)?
> In late 2017, we and Elon decided the next step for the mission was to create a for-profit entity. Elon wanted majority equity, initial board control, and to be CEO. In the middle of these discussions, he withheld funding. Reid Hoffman bridged the gap to cover salaries and operations.
I can recall the first experiments with dota2 while he was still "in charge" of openai.