It is for the same reason you do not own the story in a book you buy. You have a physical stack of paper that's yours to do whatever you want with: read it for your pleasure, prop open the door with it, burn it if you like. However the concept of the story that the book conveys – the particular ordering of the words – is not yours, it (by default) belongs to the person who created it, as they put effort into the creation of that art.

Imagine the world you are fantasizing. You spend a year writing a book, you go to a publisher to have it published. You give them a copy for consideration. You don't hear anything back and then, later, find they have taken your story and published it themselves. It becomes a top selling book. Intellectual property rights are originally envisaged to protect artistic creators from this exploitation, otherwise why would they bother to create in the first place?

(Unfortunately, there is still a lot of large organisations exploiting artists, as they largely control the media that exposes people to art in the first place. This means artists have to sign away their rights in order to get an advance and the promise of exposure.)