If you're looking for something very specific and the hiring is such that there's no time for a hire to get up to speed, vetting candidates to make sure they already have exactly the knowledge you need makes sense. But a lot of technical interviews are not really doing that. Google does not care about the specific knowledge they ask you about at interviews for the most part; they have so many Google-specific internal systems and processes you couldn't possibly have any experience with, that there is going to be some getting up to speed anyway. Instead, they're using the interview gauntlet as a proxy for something they're actually interested in, something vaguely like "technical skill".