If you are highly compensated and in a stable FANNG position, startups might not be attractive due to instability. There can also be some bias. For an example, people who want to work for startups would probably be working at one already, or be an entrepreneur. Obviously there can be exceptions.
The biggest factor will be if the person believes in the startup's idea/product. This can be them wanting a smaller company where they have more control, wanting to help a cause, wanting to work that concept/product, or wanting equity in the company that could be worth a lot.
Without those sorts of motivations, it can be hard for some startups to compete with FANNG on pay, benefits, and stability.
With shorter tenured engineers who’ve never worked outside a larger company, watch out for over-engineering. But those who are comfortable launching non-terascale solutions to non-terascale problems can typically adapt well.
People who have gone back and forth often have a good perspective and are great to work with!