As parent explained, if you use real objects, you end up testing the dependencies of those objects, and transitively the dependencies thereof, etc. For unit tes

As parent explained, if you use real objects, you end up testing the dependencies of those objects, and transitively the dependencies thereof, etc. For unit tests, the goal is to test the unit, not its dependencies. In practice, using real objects has many downsides including complex setup functions and a need to make changes to many layers of complex tests in order to implement even small code changes.