Why C++ method dynamic binding must be reference/pointer?

I know how how to use the virtual method to implement runtime polymorphism. But I can't understand why the method receiver must be pointer or reference.

From my thoughts, base class and derived class both has a pointer point to function table, When declare a non-pointer base class variable, it's easy to let that variable instance pointer member point to derived class virtual function table, like this:

class Base{
public:
    virtual void m1(){};
};

class Derived: public Base{
public:
    virtual void m2(){};
};

int main(){
    Base b = Derived(); // now b's hiding pointer point to Derived class virtual function table. 
}

This is just a language standard rule or some technically reasons? Thanks advance!