[i_have_an_idea]

> All of this machinery to filter out candidates

That's because most of the people that graduate with a CS degree can't code and, what is worse, can't be taught how to code.

As an employer, it is also very expensive and time consuming to hire someone like that, since you're not just wasting money by paying someone who's non-productive, but you're also wasting the productive time of the team (or whoever is in charge of mentoring you) in a potentially fruitless attempt to teach you how to code.

Unless you have work experience, there's no quick way to tell if you'll be able to learn or not. Faced with that problem, a lot of big companies just throw all the cognitive tests they can find at you and hope for the best.