If they had any equity they might've vested and decided there is more to life than working there.
It's hard to be very motivated to work at high pace when you can retire any moment without losing your lifestyle.
I cannot actually believe that of anyone working at OpenAI, unless the company internal culture has gotten so unpleasant that people want to quit. Which is a very different kind of change, but I can't see them going from Bell Labs to IBM in less than ten years.
I'm guessing that most key players (Mira, Greg, Ilya, etc) negotiated deals last winter (if not before) that would ensure they kept their equity even if leaving, in return for letting Sam back in.
