@liminalfiction

I think, as with the obvious and in a way correct complaints about human-shaped robots etc, this is because that's what lets the author tell the story they want to tell, whether or not it's technically maximally sensible.

Obviously people could end up visiting possible alternative worlds where basically everything is different, and those are in some sense much more likely than ones where it's the same people but in different relationships (say), but for many purposes the latter make for the best story.

In a possible-worlds scenario, after all, there's a world where ANY given thing happens, so we might as well tell the stories of the most interesting ones! :)