Proper designed modules also don't expose more than the essentially necessary on their public interface.

Failure to understand this is what leads many into microservice hell, as they try to impose a OS process wall as solution to having failed to properly design a modular architecture.

So what could perfectly be a module with direct calls, ends up being a process talking over IPC mechanisms and much higher latency, with the added fun of distributed systems.

Ironically pretty much OOP without any of the language support.