Because mods like this currently are simply not* possible.
@der_scheme pretty much nailed it. Granted, when people say that a game mod is not possible, they do not, as der_scheme tried to specify, necessarily mean that it can't be done, they're saying that it cannot be done without re-writing parts of the game's current engine...which is no simple feat. Generally, if a mod for a game requires much in the way of engine alteration, modders won't do it. Especially for a mod as relatively small as this. It simply takes too much time, effort, and damage to one's sanity. I, for one, would never do it. I simply don't have the time nor patience.
