I can't speak for everyone, but I assume most of them. Those are both rather popular and enduring stories.
Personally, I have only added to stories for which I was the owner. I rather have a story that is entirely my own instead of one I only control a small bit of.
I put a thread into Memoirs of a Minion that I remain proud of, even if I've functionally abandoned it. ... I realize that was not the question.