I'm sure he was fine financially when he first worked at Google - without leaving to found the startup as well.

But money at that level isn't about being financially secure - to have a roof over your head and food to eat - it's about power.

Money at that level gives you the ability to shape the world in ways others can only dream of - whether that be starting your own company where you can set the values, funding a cure for Malaria, or political lobbying.

Depends on whether the person in question has strong views and a strong belief that they are in the right.

Full disclaimer - I have no insight or knowledge about this particular person - just making the rather obvious and general case that joining OpenAI now at a senior level is likely to generate a serious windfall, and such a windfall is power.

As I said, no idea what motivates this particular person - don't know them at all - the money may be entirely coincidental and it's all about getting stuff done - but he did choose OpenAI rather than somebody like Anthropic....