The FAANG and similar interviews are designed—one hopes, anyway, because if they're trying to do something else they've screwed up—to select for some combinatio

The FAANG and similar interviews are designed—one hopes, anyway, because if they're trying to do something else they've screwed up—to select for some combination of decent-or-better IQ and having put in enough work to prep for their tests using, yes, online exercises and some books, with the set of things one needs to study being fairly large but also very well-known. There are lists all over the place, including one sort-of famous guide someone put on Github, outlining what you need to study, and where to study it. Basically they want you to be at least semi-smart in a raw general intelligence sense, and to really want the job and have put in the work to get it. Whatever their reasons for this, it will almost certainly have the effect of making a successful candidate value the offer more and think more highly of their co-workers, for the same reason that e.g. initiation (hazing) rituals or costs tend to improve group cohesion. Perhaps this is why they do it.