> I am not even allowed to tell a candidate (another human being that probably NEEDS a paycheck) why I didn't hire them and what they can do to improve their viability

I really like interview questions that have binary results. If you can do X, you can do this job. That way if a candidate fails, they pretty much know why they failed. They couldn't do X. It's not always possible to phrase questions like that, but when I can, I do.