There are a number of reasons people may care. For instance, the thing about art that appeals to me is that it's human communication. If it's machine generated, then I want to know so that I can properly contextualize it (and be able to know whether or not I'm supporting a real person by paying for it).
A world where I can't tell if something is made by human or by machine is a world that has been drained of something important to me. It would reduce the appeal of all art for me and render the world a bit less meaningful.
Fair, but I think that will shake out easier than expected: if there is a market (i.e. it is being valued) for certain things human generated people will work on being able to authenticate their output. Yes, there will likeky be fraud etc., but if there is a reasonable market it has a good chance of working because it serves all participants.
A world where I can't tell if something is made by human or by machine is a world that has been drained of something important to me. It would reduce the appeal of all art for me and render the world a bit less meaningful.