"We don't want you to know you've already failed so we can waste your time for a month in the off-chance nobody better shows up."
If someone fails my interview, it is completely possible that they’ll still get an offer. I have one data point, and my colleagues are going to collect more. I’ve changed my vote from no to yes in debriefs many times once I saw other feedback. But that’s a lot less likely to happen if my interview wrecks their confidence.
1. You don't know what metrics you're being judged on.
2. You do know but you either can't assess your abilities in said metrics or you can't tell how well you've presented them.
The second one is up to the candidate, not much you can do about that, but if during an interview you have no idea what the interviewer is looking for, that's a terrible interview.
I’ve already explained why I think obscuring poor performance to preserve candidate confidence is crucial. If you think that’s a “terrible interview”, maybe you could elaborate on why, rather than just asserting it.