Regarding your desire for explicit closure: I don't personally feel that explicit closure is actually possible in most cases. I will give you my personal "case studies" to help illustrate my thought process, since I seem to be the kind of person you don't understand.

I had a friend who was flakey and unreliable. I stopped contacting and responding to him after we agreed to meet somewhere and he never showed up, and gave no explanation. My thought process, insofar as I explicitly reasoned it out, was: The emotional cost (and "status" cost) of further incidences like this is greater than any conceivable value this friendship may have had. I do not want to repair the friendship, so there is no point in telling this person what they did wrong. I have no realistic hope that they will amend their pattern of behavior. So, I will terminate all contact without explanation.

I've had at least a couple of friends with whom interactions became increasingly argumentative and critical and decreasingly positive and fun who I just stopped responding to because I could think of no affirmative reason to respond.

I had a friend who I discovered had been very deceptive towards me. In this case I told her why I was terminating contact and then terminated contact. Frankly the only reason I told her the reason was because I was angry and wanted to hurt her feelings. My normal impulse would have been to just "disappear."

It is interesting that both you (magfrump) and I seem to both be committing Typical Mind fallacies in how we expect other people to react to our actions. I see ceasing contact without explanation as the default course of action, and you see providing an explanation as the default course of action, and we misunderstand other people who have different default responses. I see now that my policy in the past has not been rational. Whether I am capable of meaningfully updating on this is a different question.