I've been thinking about this as well, and I think it becomes fairly clear why they do it if you look at it from a BigTechCo's perspective. From Google's persp

I've been thinking about this as well, and I think it becomes fairly clear why they do it if you look at it from a BigTechCo's perspective. From Google's perspective (or whoever), they're about to sink a million dollars into a new hire. I'm getting this number from salary, stock, benefits, other engineers time, etc. They won't see a return on this investment for 6 months to a year, because it takes a long time to learn their obscure, complicated tech stack. They want to be sure they're getting someone who's willing to grind on "pointless" tech problems, because that's what they'll be doing for their first year there: learning the tech stack.