A major reason why this is NOT already implemented is simply that mIRC is designed to provide you with the tools and techniques to be able to come up with your own, as well. The main issue here is, like praetorian_ suggested, leaving the features up to you.
Another good reason is that if Khaled starts moving on with something like taking shadows or depth into his own hands, people are going to start requesting rendering engines and whatever, and that is just waaaaay out of the ordinary and out of contrast to what an IRC-client, with a damn long and fine list of scripting capabilities, is supposed to do.
Khaled created DLL's to get around people requesting off-the-wall features, but some have been added anyways. Let's just hope that someone gets to creating better extensions for /draw<ing> :-)