Why doesn't polymorphism work without pointers/references?

Question

I did find some questions already on SO with similar title- but when I read the answers they were focussing on different parts of the question which were really specific (e.

Answer 1

I found it really helpful to understand that a copy constructor is invoked when assigning like this:

class Base { };    
class Derived : public Base { };

Derived x; /* Derived type object created */ 
Base y = x; /* Copy is made (using Base's copy constructor), so y really is of type Base. Copy can cause "slicing" btw. */ 

Since y is an actual object of class Base, rather than the original one, functions called on this are Base's functions.