It's because these emulators are not faithfully recreating the Wii's GPU. Instead, they take intercept the rendering commands ("draw this polygon", etc.) and render those commands in whatever resolution they want to.
This means no upscaling is needed.
It's usually called High-level emulation (which started with the Nintendo 64 emulators if I recall correctly). It needs tuning for some games, but it has performance and picture-quality advantages.
PS: BTW, this is also why the 360 improves the graphical quality of Xbox games with backwards compatibility. The games are actually rendered at a high resolution, not just upscaled.
It's because these emulators are not faithfully recreating the Wii's GPU. Instead, they intercept the rendering commands ("draw this polygon", etc.) and render those commands in whatever resolution they want to.
This means no upscaling is needed.
It's usually called High-level emulation (which started with the Nintendo 64 emulators if I recall correctly). It needs tuning for some games, but it has performance and picture-quality advantages.
PS: BTW, this is also why the 360 improves the graphical quality of Xbox games with backwards compatibility. The games are actually rendered at a high resolution, not just upscaled.