In distribution, first of all, there is an overhead cost in communicating between systems.

Technology is very effective in communicating within a single process. There is effectively no overhead in calling functionality that is part of the application. As soon as inter-process or remote communication is required, engineers have to define interface abstractions. Communication protocols such as HTTP have to be defined and used in order to exchange information.

This requires certain time and effort. Communication between applications has to be defined, implemented, and maintained. Within a single application, the communication is reflected in method invocations.

The required communication also becomes a concern of the business use case. It can no longer be assumed that certain functionality or data can be just used without any overhead. Communicating with the distributed system becomes a responsibility of the application.