Because the experience in the interview bears virtually no resemblance to the daily requirements in the job. This means that your success (or failure) in the interview is not a good indicator of your success (or failure) in the position.
To get more specific and to pick one of those companies, Amazon’s turnover rate is sky high, but they’ve chosen to optimize by hiring at an insane pace to compensate (rather than to optimize on improving retention), which means their hiring practices are optimized for volume, not quality.
This is true for almost every interview I had in the past years, they never resemble what you actually do when you are able to go in. There job descriptions that requires many technologies they don't even use either. It is not particular from big companies.
I've heard about that although there divided opinions about Amazon specifically, it looks an awesome place to work and I've heard contrary opinions as well. In the end it depends, like anything else...
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