They probably get a ton of those and just keep them on file until needed, re-date and you’re good to go. 
I know that at least some of the Ann Landers/Dear Prudence ones are real, because there have been at least two or three recent instances where the same letter got selected for answering by more than one of them, with interestingly different answers depending on the columnist.
I’m pretty sure at least one of them got noted here, but I’m not having any luck searching for it (no great surprise, that is).
I would think that it would be a lot more ethical for a publication to “edit submissions for clarity” and or house style, rather than simply make them up out of whole cloth.
Ethical? The answers are important; the questions that give rise to the responses are meaningless.
Jokes have been repeated, with long intervals between, probably by different people. I once sent in a letter to the editor of a newspaper, to find it re-written as an editorial. I didn’t know whether to be flattered, or indignant.