When I have hired people, I have always offered to explain the reasons for rejection, when I turned people down. The ones that opted in, I have given an answer. This way, I don't have to spend time on telling people why I turned them down if they don't really care.

The thing is - It's quite hard to give that kind of feedback. A great deal of the assertion of candidates ultimately comes down to instinct. That's hard to qualify. Besides, you want to be very careful about your wording, so it usually takes up a lot of time to write even a few paragraphs back to a candidate.