An ex-Amazon engineer at a top tech company told me he felt that his current colleagues were higher-caliber than his colleagues at Amazon, and that many candidates that were rejected would have been accepted by Amazon. So that's a possible answer.

That being said, Amazon is still considered a top tech firm, even if it is a cut below Google/Facebook in prestige. There aren't many tech firms as well-known as Amazon. I can see many fresh grads going to Amazon for 1-2 years just to get it on their resume.