Why don't non-const lvalue references bind to xvalues?

The following does not compile:

#include <iostream>
using namespace std;

int x = 5;
int && f () { return std::move(x); }
int g(int & y) { return y; }

int main() {
    g(f());
    return 0;
}

It's clear to me why non-const lvalue references do not bind to prvalues (unnamed temporaries) -- it does not make sense to modify them, as they will soon disappear. Yet why do non-const lvalue references not bind to xvalues?

If a function returns int &&, the referenced object can't be temporary, otherwise we would get a dangling reference. Hence if an int && is returned, that's, in my understanding, a reference with the additional guarantee that it's safe to move from it.