Not only that, consider the situation now, where Sam has returned as CEO. The ones who didn't sign will have some explaining to do.

The safest option was to sign the paper, once the snowball started rolling. There was nothing much to lose, and a lot to gain.

People have families, mortgages, debt, etc. Sure, these people are probably well compensated, but it is ludicrous to state that everyone has the stability that they can leave their job at a moment's notice because the boss is gone.

Didn’t they all have offers at Microsoft?

I think not at the time they would have signed the letter? Though it's hard to keep up with the whirlwind of news.

They didn't actually leave, they just signed the pledge threatening to. Furthermore, they mostly signed after the details of the Microsoft offer were revealed.

I think you are downplaying the risk they took significantly, this could have easily gone the other way.

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.