Why cant I used a function returning a pointer as a lvalue?

For example this one works

int* function()
{
    int* x;
    return x;
}

int main()
{
    int* x = function();
    x = new int(9);
}

but not this

int* function()
{
    int* x;
    return x;
}

int main()
{
   int* x;
   function() = x;
}

1. While I can use a pointer variable as a lvalue, why can't I use a function returning a pointer as a lvalue?

2. Also, when the function returns a refernce, instead of a pointer, then it becomes a valid lvalue.