This feels like a retcon. The real reason is most likely that Google started interviewing people this way and then that's the culture that developed in the industry. Once a culture develops, it's very hard to change.
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.