It's a design decision for the reasons already mentioned by others. What's important to realise is that the same concept applies to other systems in the game as well. Things such as good are abstracted, meaning that they generate movements in the economy without the actual underlying "things". This is also why the stockpiles don't exist. They simply can't, because there are no "physical" goods to stockpile. It's an abstract system of supply and demand to generate price fluctuations. Once again- whether someone likes it or not, it's a deliberate design decision.
Similarly with the buildings. Some people ask for certain building types to be constructed at province level. This goes, once again, against the very framework of the game's mechanics. Buildings cannot be constructed at a province level, because they are state-level abstractions.
Yet another example- POP cultures and their representation on the map. There have been complains about certain groups not being represented on the map, but that's because of the level of abstraction that the game implements. POPs are state-level concepts, so it's simply not possible to accurately show POPs at province level.