They're leaving because (and this is a vague recollection of some articles I read online so might be incorrect here) is Altman seems to have unrealistic goals. Could also be that those heading out don't want to be tied to any copyright suits. Or as the 2nd commenter said, money.
Or, they only went there to learn enough about how OpenAI is doing their magic and want to go chase those sweet VC dollars with their own startup.
