First, there are strong diminishing returns to well-being from wealth, meaning that moving oneself from the top 0.5% to the top 0.1% of global income earners is a relatively modest benefit. This relationship is well studied by social scientists and psychologists. Compared to the potential stakes of OpenAI's mission, the balance of importance should be clear.
Two, employees don't have to stay at OpenAI forever. They could support OpenAI's existing not-for-profit charter, and use their earning power later on in life to boost their wealth. Being super-rich and supporting OpenAI at this critical juncture are not mutually exclusive.
Three, I will simply say that I find placing excessive weight on one's self-enrichment to be morally questionable. It's a claim on human production and labour which could be given to people without the basic means of life.
OpenAI employees are as aware as anyone that tech salaries are not guaranteed to be this high in the future as technology develops. Assuming you can make things back then is far from a sure bet.
Millions now and being able to live off investments is.