Stock options usually have a limited time window to exercise, depending on their strike price they could have been faced with raising a few hundred thousand in 30 days, to put into a company that has an uncertain future, or risk losing everything. The contracts are likely full of holes not in favor of the employees, and for participating in an action that attempted to bankrupt their employer there would have been years of litigation ahead before they would have seen any cent. Not because OpenAI would have been right to punish them, but because it could and the latent threat to do it is what keeps people in line.