Problem is for a company like Amazon, even if the brain drain will result in obviously inferior customer experience, it could take years before that happens and for it to be recognized and for the business results suffer for it. In the meantime, bigger margins and restricted stock matures and they can get their money now.

Particularly with business clients, like AWS customers, it will take a huge amount of obvious screwups before those clients are willing to undertake the active effort of leaving.