Because those aren't 1:1. Someone who can't play football can still play Fifa, someone who can't skateboard can still play Tony Hawks, someone who can't race can still play Gran Turismo.
They may have the same 'theme' as the real sports, but they play completely differently. They are a different experience to the real thing. The same goes for all the Wii sport titles - Wii golf doesn't play like real golf, Wii boxing isn't like real boxing, Wii tennis doesn't play anything like real tennis, and so on and so on.
What you seem to want is dodge ball, only with a virtual ball instead of a real one. If the game mechanics are no different to the real thing, what's the point? What actual value would Natal dodge ball have over real dodge ball?












