It's a cultural thing. I've seen code bases where every class was an FooImpl and had an associated FooIf, all in name of decoupling and mocking. Though it may be a trend that's dying down.
You cannot eliminate complexity (although you can minimize it, to be clear). But for most things, you can shuffle the semantics around, you can paper over them in a new way, but at the end of the day, someone has to pay the piper.
In my experience, complexity eliminated from one spot will invariably bubble up somewhere else in an unexpected way.
It's still possible, mind you. TypeMock has been offering this exact ability for C# for many years now. But the free TDD frameworks generally didn't have this.
This is not necessarily zero-cost however - if the compiler cannot prove specific type members being invoked, it has to construct an execution profile and then apply it to subsequent compilations, and also emit a guard when doing dispatch on those.
[0]: https://github.com/dotnet/runtime/blob/main/docs/design/core... (note - subject to change)