(see notes at the end before flaming)
Amazon interviewers typically go through a long "shadowing" process where they calibrate their questions and evaluation criteria. You can't just go on to interviewing candidates on your first week. Also, interviwers interview for their own "job families", so e.g. SDEs and SDMs will typically interview technical roles and likewise for non-technical roles. All interviewers are also required to enter detailed "feedbacks" (if you've been put-off by the interviewer furiously typing away, please pardon them - they're not emailing, just taking detailed notes).
Amazon bar-raisers go through additional rigorous shadowing process that typically lasts many dozen of interviews and calibrations. An Amazon BR is more like a "moderator" in the interview loop than an all-powerful veto-machine. Veto is a very powerful tool, to be wielded extremely sparingly. BR on a loop is an "external" third-party who makes sure no self-interests are in play while hiring a candidate - i.e. no team is compromising on quality just to fill open positions.
Also, it's patently false that BRs ask extremely hard questions. Infact, most questions of BR focus on soft-skills and Amazon's leadership principles (Google them, we take them very seriously!). Also, BR's do loop within their job families and anything otherwise may happen only under extraordinary circumstances.
Notes: Amazon being as large it is, is comprised of thousands of autonomous groups and hence some candidate's experience may be sub-par and might have been contrary to what I've written. If you had a bad experience while interviewing at Amazon, please reach out to your recruiter who'll then forward your feedback to BRs. We take instances of bad candidate experience very seriously and will try to fix deficiencies if any. While recruiting, we consider the candidate as our customer and strive to make their interviewing experience delightful.
NB: Yes, I'm an Amazon BR. Posting using a throw-away account.