It's an arms race. Promotion reviewers expect some portion of candidates for promotion won't pass promotion - otherwise the bar isn't being set high enough. The answer to this is that managers refine the promotion packets to ensure their employee's promotions are in the upper bracket - which in turn forces reviewers to recalibrate.

Eventually you are left with an over the top expectation of what a promotion case looks like. As an engineer at a FAANG I recently moved out of a team where I had built a range of services and was close to PE, because I found that I was closing in on a point in my career where I was spending ~80% of my time on promotion oriented activity... which in turn means politics.