The premise behind flight risk money is that people might not be interested in optimizing their salary by constantly switching jobs, especially because they might really like their current job. But a lot of people have a short list of jobs that they would be interested in if money weren't a factor; many employees (maybe most) aren't in their absolute ideal job, but are balancing money and work life. When you give those people lots of additional money, you change that balance.
This isn't complicated, right? A pretty large group of people (1) like their current job, and (2) would like to start or join a new company.
That's just not the same as what happens with executives. Executives are not waiting for their big break so that they can start a new thing.