Sorry nzeeman but you are still totally wrong, it is as retinax explained and nothing more.
As to the original question- There is more to design besides distortion and resolution.
Light fallff and color aberrations also increase dramaticallt towards the image corners. If you need a lens to cover "full frame" you have more limited design choice as you suspect "corners would go wild" but in many different ways. A lot of lens elements are added in fast lens designs just to maake sure that corner performance is acceptabe too.