With comics, I've always heard that it's because (at least originally, when most US comics were read by kids) the comic book audience was expected to turn over every five years or so. No one expected audiences to read for ten or twenty years, or save their old comic books. Plus they were most often work for hire, so the creative teams changed a lot. They were an ephemeral medium. I think TV has similar factors at work. Plus with books and movies, you generally know your whole story ahead of time. In TV and comics they're more likely to be making it up as they go along.