For largely the same reasons why graduating from top schools is given more credit than any old college, even if the level of teaching is similar. Getting in takes determination, intelligence, and discipline. Same with getting good at Leetcode.
You'll be a small cog in a large machine, engaging in theatrics justifying in length why your 10h/week of serializing/deserializing JSONS was so difficult and important to the company, while you rest and vest playing videogames at home, asking on Reddit if everyone working in FAANGs is as bored as you.
It is very hard to change established processed - especially when the people doing the interviews have gone through the process.
They had to suffer, so why should the people after them get an easier ride?
People in the middle ages and WW1 had it pretty bad, why should people after them get an easier ride?
Because humanity progresses when we make things easier for the next generations instead to clinging to the 'crabs in a bucket' mentality of perpetuating the cycle of collective suffering and suckage.
It's an extremely poor predictor of a candidate's quality.
It should only be used in those companies where the number of applicants is way larger than the number of open positions where you can accept the trade off of losing few great applicants if you can also lose the many more bad ones.
Just because you say those questions are an extremely poor predictor of a candidate’s quality doesn’t actually make it so. It might just be one of the best and most cost effective ways of finding good candidates.
It's a way to get rid of many false positives even if you lose some false negative.
Again, companies like Google can afford to lose amazing devs that can't be bothered to farm leetcode if the trade off is getting rid of those that would not be able to farm those effectively.