Earlier quoted context omitted.
You're mistaken in that you believe that the goal of culls is to cut underperformers. Stack ranking doesn't assess performance, it checks whether you're willing to do a little more to be ahead of your peers. The real goal of culls is to motivate people to work more, to be more accessible to their managers' requests, etc. so they make it "to the next round". It's a pressure tool, not a performance control tool.
This sort of thing is one of the big reasons why I would never work for a FAANG company. I prefer to work at a place that encourages good work rather than a weird cutthroat arena combat thing.
However, on the bright side, it is kind of nice that FAANG attracts the type of people who actually think this sort of constant stack ranking arena fighting is okay. Think about it: if they really hated it, they'd leave. Obviously the pay is a huge motivator, but if it was as distasteful as it was to many others, they wouldn't hang around.