Then you have to reimplement method call count when you need to assert something was called in your interface-based mock; and reimplement differing returns base

Then you have to reimplement method call count when you need to assert something was called in your interface-based mock; and reimplement differing returns based on input or call count; and maybe reimplement throwing exceptions when those are necessary to test not-so-happy paths. In the end you'll have an implementation of the interface with quite complex logic in the methods' implementation to handle differing returns, which will couple your mock implementation to the test cases setup, or you'll reimplement a mocking framework. I prefer to use a mocking framework to deliver stuff instead of stabbing myself to keep purity of interface-based APIs.