It's quite simple: the talent pool that had already enough money that they quit their well paying job at a for profit company in part because they wanted to continue working at a non-profit high impact.
As OpenAI found its product-market fit, the early visionaries are not needed anymore (although I'm sure the people working there are still amazing)
I think OpenAI took this play right out of one of its founding donors playbooks. Pretend your company has lofty goals and you can get people to compromise to moral relativism and work superduper hard for you. These people definitely have framed posters with the “If you want to build a ship, don’t drum up the men to gather wood, divide the work, and give orders. Instead, teach them to yearn for the vast and endless sea" quote somewhere in their living places/workspaces.
As OpenAI found its product-market fit, the early visionaries are not needed anymore (although I'm sure the people working there are still amazing)