The Wii is just not some magical device that allows 6-year-old tech to perform like 3-year-old tech... sorry.
The Wii will never have a game on par with CoD4, plain and simple. Its okay -- the Wii wasn't meant to compete with the HD consoles. Plenty of last-gen era games look pretty damn good (e.g. God of War 2, SotC, Fable 1, Halo 2, Metroid Prime 2, etc.), and the Wii should be able to outdo those games by at least a small margin. Leaping forward to CoD4 is too much though.
Contrary to popular belief, this is not a problem.







