This issue i think also stems from that idea that no one really likes 'unfinished business' as it were. You create a story with loads of potential for growth, but something happens that prevents you from completing your vision, usually because of work or something that interferes with you creative processes, or like in the case of films or TV shows, sometimes shows are cancelled prematurely or films that set up the potential to explore things in sequels end up not having any sequels.
I've seen plenty of fanfiction stories where writers have created something truly awesome that engages your interest and your imagination and then you reach a point where there is no more story because the writer has not gotten around to continuing it, or they have simply lost interest in continuing the story and it just sort of hangs there for years as an incomplete work.
its hard to know where this need for closure comes from, but I guess its that idea within fiction that since there is a start to a story you naturally would like to see that story through to its conclusion. but it also then becomes an issue of how satisfying that conclusion becomes if someone else takes up the threads of the story rather than the actual person that conceived the story, kinda like the V mini series from the 80s and the follow up V: The Final Battle, and the short-lived V television series that followed on from both mini-series. The original mini series was created by Kenneth Johnson, but the follow ups did not have Johnson involved at all. Yes, the story was continued based on what he started, but its hard to know if the follow ups are what he would have done had he actually been involved.
its the same when there are those who create virtual continuations of short-lived programs, the writers obviously try to create something to continues the story and provides closure, usually by trying to come up with what they feel would be a natural continuation from how something ended, and also try to craft something that honors the same spirit and atmosphere of what was created, but in those cases it is based on the writer's imaginations rather than the actual intentions of the original creators. ]
However, it is also true that very often someone that has a great idea can only see so far, and while they do lay out the blueprints and the template and the groundwork for the universe created, sometimes you need others that can inject fresh new blood and new ideas in order keep that universe thriving, kinda like with Star Trek and more recently with Star Wars.