It could definitely be possible, but the problem with WINE is that almost every windows application requires an exact, specific version of that application for it to work. For instance, I tried installing EverQuest on it under Ubuntu awhile ago back when I was into Linux, I needed to install EverQuest by going online, finding the old specific client I needed, installing it, then editing the client to be compatible with WINE. I also needed to do that for a couple other applications. As you can see, it would be both expensive and time consuming for Apple to have WINE in Mac OS X seemlessly.
And, if Apple were to enable WINE etc., developers would stop developing Apple only software and make Windows software only, because you can reach two huge market OSes with one software package without having to rewrite it.
Besides, if you want WINE under Mac OS X, there is always Crossover.