Altman and OpenAI’s other founders rejected Musk’s proposal. Musk, in turn, walked away from the company — and reneged on a massive planned donation. The fallout from that conflict, culminating in the announcement of Musk’s departure on Feb 20, 2018..."
Now why are you being obtuse on ignoring another real reason, Elon Musk was poaching people from OpenAI and argued for a conflict of interest.
"Elon Musk leaves board of AI safety group to avoid conflict of interest with Tesla" - https://www.theverge.com/2018/2/21/17036214/elon-musk-openai...
From all the HN favorites memes, the two most strong that need to evaporate, are that Elon Musk wants to save humanity, and Sam Altman does not care about money...
And yeah, Sam cares about money and some other things, it seems.