For variety of reasons. First, if an uploader sends a file that is malicious, it can’t be taken down since it is not moderated. Second, I could create a torrent for “Oppenheimer” and name the file as the same with proper meta data but upload a fake one. It can’t be removed since there is no moderated. Also, the chances of people uploading “bad” content are high too, though I don’t know whether people do that though.