The uploader is not really meant for socializing - it's just a tool to upload files. It facilitates file sharing in places that don't have built-in upload functions, don't support certain file formats, and don't support large file sizes

Adding a reply function would expand it's scope too much and significantly increase the amount of moderation required. It's better to make a post on the appropriate area of the site and paste uploader links instead (which is how it's meant to be used)