Sure, they can, and yeah it is pretty easy, but people have lives. They move on. A distro always has someone checking to make sure things aren’t broken. On Flathub it won’t even break. It’ll just waste drive space and start giving users annoying error messages, and there if the maintainer isn’t interested in maintaining it anymore the only option for doing anything about it is to fork the whole project, and who’s going to do that for something that isn’t even really broken?