If you were to ask me, I would say that middle managers are generally miserable and are trying to avoid risk and maintain job security by NOT hiring people as long as there is VC money to burn. But I'm a nihilist.
I know this is late, but a lot of us really want to hire people, but there are a lot of people out there that are overconfident in their skillset, and sometimes you have to do a quick sanity check to weed them out. I agree, though, that most you can tell before that point. I'd never make someone donate a day to a test, but 15 mins of whiteboarding can go a long way if someone doesnt have a lot of publically available code to go look at (and, a lot of devs don't, due to working on public repos)