1. The highest paid executives in other industries tend to be at the apex of their careers, and new startups would be a status step down for them.
1. Empirically, executives do switch companies somewhat routinely.
Your last sentence presents a false dichotomy: either we're meant to reject the notion that flight money is a real phenomenon, or we're required to believe engineers shouldn't be compensated above slave levels. There's a huge excluded middle in that argument.