Separating the UI from the functionality is not as easy as it sounds. Often the first iteration of a program has the business logic tightly coupled to the UI, and is designed for a single platform. You need engineers with foresight to refactor before it becomes a mess

Also even the non UI is not trivial. I once worked on a very la4ge code base that was console only. We en we eventually gave up on our Windows and Mac ports. Granted it was C++ …