Why do we require requires requires?

One of the corners of C++20 concepts is that there are certain situations in which you have to write requires requires. For instance, this example from [expr.prim.req]/3:

A requires-expression can also be used in a requires-clause ([temp]) as a way of writing ad hoc constraints on template arguments such as the one below:

template<typename T>
  requires requires (T x) { x + x; }
    T add(T a, T b) { return a + b; }

The first requires introduces the requires-clause, and the second introduces the requires-expression.

What is the technical reason behind needing that second requires keyword? Why can’t we just allow writing:

template<typename T>
  requires (T x) { x + x; }
    T add(T a, T b) { return a + b; }

_{(Note: please don’t answer that the grammar requires it)}