When a image is posted on this site (and if it hasn't had the EXIF data stripped away by an image editing application) the EXIF data will show up.
But because the file size of an image posted to these forumsis limited (250400 bytes), a lot of people post their photos elsewhere (i.e.: smugmug.com, flickr.com, etc.) and include links to their images in their posts here. When they do that, you can't get the EXIF data with IExif.