Isn't that clear by now? People want to get into the most famous companies. They think that by osmosis they'll be better engineers themselves, it looks good on the resume, they may be exposed to some brilliance, and, surely, once they're there, these companies will recognize their mistake and promote them into their rightful position. Not to mention that it's notoriously hard for people to self-assess their level, also because the coding gods of one company can be the low performers for another company. And personally, I have no reason to believe that Amazon, Facebook, Google or Microsoft are necessarily on either one far end of that spectrum.