Because it is. Everybody knows these companies get to skim the cream and they vet their candidates thoroughly. As a start-up with no HR and poor hiring processes it, more often than should, gets the interview/job.
Note that I would work for Apple and I will not work for google, because google is not high enough quality to meet my standards. (And I personally saw Mark Zuckerberg say "never hire anyone over 30 years old.. they just don't get it" at a YC Startup School in 2007 or 2008, so I'm of the impression they probably are close to as bad as google at being able to tell who is good and who is not.
Being able to hire good engineers is not an easy task, because you have to be an excellent engineer to be able to tell who is good and who isn't.
I have the distinct impression that people in their 20s do not realize that these companies names on their resumes are only feathers when presented to lower quality hiring departments.
If you want to work with the best of the best, you're not going to rely on the fallacy of correlation.
Secondly, you have no data to back up this claim:
> (I'd put myself at %80). The average Amazon engineer is %40.