They really can't, because a majority of their compensation is in now-unreasonable amounts of OpenAI equity (because it was worth much less when it was granted). If employees decided to walk away, rendering their vested equity worthless, they wouldn't get anywhere near the same value of, say, Microsoft equity as a signing bonus to make it worth.
Any senior OpenAI employee with decent amounts of vested equity on low valuations has strong financial incentives to stick with the company, and also strong financial incentives to convince others to stick with the company.
Some more thoughts from back when employees "threatened" to leave: https://www.businessinsider.com/openai-employees-did-not-wan...