Hard interview questions are asked for the same reason hard questions are asked in SAT tests. The assumption is that if you can do a hard thing in the interview (implement a mutex using a semaphore or whatever) then you can probably write simple code too.
A few years ago I was working somewhere that had a low hiring bar. We were using an opensource graphql library to interact with some service. The graphql library didn’t expose some queries we needed, so I forked the library and added those queries.
After I left I heard through the grapevine that they were very unhappy with my work. They didn’t have anyone on the team who was skilled enough to maintain the code I wrote, so I left them in a bad place.
In retrospect I’m still not sure what else I could have done to solve the team’s graphql problems. Not that, but what?
Having said that, I know great, intelligent programmers, sone of who worked on compilers, who are simply bad in a whiteboard setting. Having a filter is necessary, I’m just not convinced a Google-style interview filters exactly the right thing.
The low hiring bar is relative. If all you do is put together crud apps and it's mostly stitching things together you might not even need a formal CS education. In fact a lot of people I know are wildly successful because they found their niche. They would have an exceptional bad time in a whiteboard setting.