Well, it isn't really used for DRM content decryption anymore, and Vista users have the Windows Media Framework to fall back on if ffvpx fails them in some way (like the Twitter audio bug). So if you're on Vista, I can't see why you would need or want it. But it is useful on XP.
Since the same browser is used by users of both OSes, it probably makes sense to keep it separate. That way XP users can install it if they need it, without bloating the package for Vista users.
Besides, like any add-on it gets installed in your profile folder, not the main program folder, so it would be tricky to include in the browser package anyway.