Not just Sam, since Greg stuck with Sam and immediately quit he set the precedent for the rest of the company. If you read this post[0] by Sam about Greg's character and work ethic you'll understand why so many people would follow him. He was essentially the platoon sergeant of OpenAI and probably commands an immense amount of loyalty and respect. Where those two go, everyone will follow.
[0] https://blog.samaltman.com/greg