[crearo]

FWIW, every data structure / algo interview I've done allowed me to use built in functions for sorting (I work at FAANG).

In general, I dislike the interview process too, but there just isn't a better way to do these, as is clear from this large thread. If you look at it from the company's perspective, you have to conduct interviews at scale, and it can't hand off the same system problem to everyone.

You're being tested on your problem solving ability. It doesn't correlate directly to your job, but the general assumption is, if you can solve puzzles after studying for them, the chance that you can solve any random problem thrown at you is high.