Why s std::hash not specialised for std::reference_wrapper?

I thought it would have been, but I can't find this in my standard library implementation (gcc-4.8.2).

Why is std::hash not already specialised for std::reference_wrapper?

#pragma once
#include <functional>

namespace std 
{
    template<typename T>
    struct hash<reference_wrapper<T>>
    {
        size_t operator()(const reference_wrapper<T>& r) const
        {
            return std::hash<T>()(r.get());
        }
    }; 
}