Why can the using directive not be used recursively in c#?

Question

Why cannot the using directive be used recursively in c#?

using MyKey = System.String;
using MyValue = System.Int32;
using AllowedPair = System.Collections.Generic.KeyValuePair<System.String, System.Int32>;
using NotAllowedPair = System.Collections.Generic.KeyValuePair<MyKey, MyValue>;  //CS0246

I felt it was reasonable to want to build up using statements in this way so I am curious why I am not allowed to.