Really, for the most part, the talent goes to the best places to work. That's subjective but the major factors for most people are mission (e.g. charity good, privacy violation bad), working environment and compensation.

Amazon isn't really particularly good at any of these, so you'll tend to find more talent at places that do better. Facebook and Google both have better compensation and from what I've heard from friends (who've also had experience with Amazon), both also have a better working environment.