Back then he said it was because of "conflict of interest" with Tesla. But that wasn't entirely true, he wanted to be the boss and run the company. Altman said no. [citation needed]
> That's my speculation. If I was Musk, I wouldn't voluntarily quit OpenAI.
In hindsight. Back in 2018 it wasn’t at all obvious they would be in the position they are today. Google released the Transformer paper just a year before and were commonly seen as being in the lead.