Sam was brought back because there was no one to replace him. The non-profit types on the board were living in a consensus bubble that didn't extend far beyond a small inner circle, and they discovered that they didn't have sufficient support from the engineers who had lots of other employment options and threatened to quit if Altman wasn't reinstated. Altman himself had no problem finding a replacement job in a matter of hours, and the board was looking at a business drained of talent in a cut-throat tech race.

I'm no fan of Altman or OpenAI, it's a pretty shady company and I am suspicious of their books, but this was a great demonstration of the uselessness of boards and how out of touch they are with the business they are supposed to be supervising. It's really rare to find an effective board, primarily they sit like a House of Lords enjoying ceremonial perks and a stipend in exchange for holding a few meetings a year.

Because he's a charismatic liar. Extremely effective and useful for a company that is burning money to secure more investments.