I've been on both sides, interviewer and interviewee. I always give feedback if asked, and endeavor to be scrupulously fair.
My take is that it's all pretty arbitrary, at the end of the day. Most interviewers do not 1) spend time putting the candidate at ease, 2) ask the same interview questions across candidates for the same position, 3) really introspect about whether they are doing it well.
Interviews are a notoriously terrible way to get a good grasp of how someone will do on the job, and most interviewers in my experience do not recognize that. The more an interviewer is convinced they are good at it, the more they really just suffer from Dunning-Kruger. No interviewer ever feels like they have failed when they reject a candidate, although it's very well possible that they did, and badly.
