In my opinion, it's infantile bro culture hazing and feelings of inferiority. Oh, you can't traverse a binary tree using Breadth-first search in aarch64 assembler? You obviously aren't qualified to work on our web application.
I've applied for front-end jobs and get asked some C.S. puzzle, to do something no front-end developer has ever needed to do (unless they wrote the one module everybody else uses).
And yet, it's not like I couldn't grok that if I needed to... just pay me and I'll figure it out, no problem.
The difference between me and the applicants they are filtering, is that I can both learn and implement w/e algorithm, in any language you want. Want proof? Just look at all these other algorithms I learned and wrote.