Well...there's a lot of reasons...but let's shift focus from music/movies to something like photos. You can imagine how arduous it would be to maintain a database like that that even if possible from a technical standpoint, would not be an undue burden upon photo sellers/buyers. Plus, a one-stop centralized database of everyone who owns everything from "Casablanca" to "Debbie Does Dallas"? That's something that might give the database-fearing crowd some pause.