Too much work. A short time before the Megaupload bust another one-click-hoster sued a media company because they felt the studio was misusing their privileges. Apparently the media companies had special access and were able to directly delete files on the servers and this media company were constantly deleting instances of one specific free software. Sadly no more facts ever were known because the one-click-hoster self-imploded after the Megaupload thing.
It was speculated, that the piece of software carried the same name as one of the studios properties (The Firefox browser was suggested).
So no one at the media company was able to do a cursory examination of the files in question and could see, that a 30 MB exe-file is clearly not a pirated movie.