There were reasons which I cannot seem to remember right now. Fan noise maybe? But why dont cases blow air up?
Actually modern PC cases such as Bitfenix's Prodigy/Phenom, Fractal Design Define R, and specially made for air cooling Corsair Carbide Air provide vent and filter on top. You can install apair of large fans on them but most of the time, they are used for watercooling kit.