Because you don’t have to operate your 11" MBA with a 2.5" by 0.5" sized mouse cursor (your finger). If you move your tiny tiny mouse cursor on a high resolution screen and use a lot of keyboard shortcuts then it might work. If you start to attach mouse and keyboard to a tablet then you don’t need a tablet.
I’m not saying that a regular Windows 8 is useless. I’m just saying that some people here overestimate the need for such a tablet by far.