Why can't an interface have class methods?

Why can't an interface have class methods?

How cool would it be to do:

type
  ISlice = interface
    class function New(Arr: TArray; Start, Slide, Size: integer): ISlice;

var
  IntSlice = ISlice;
begin
  IntSlice:= ISlice.New(Arr, 1,2,10);
end;

No need to know about implementing classes, unless you want a special flavor of ISlice.

Of course this does not just relate to construtor like class functions, but any sort of class method.