> They share a currency. So do the US and Zimbabwe. The issue is only partly economic, it's also cultural. Europe's countries are far more culturally varied

> They share a currency. So do the US and Zimbabwe. The issue is only partly economic, it's also cultural. Europe's countries are far more culturally varied than US states; different opinions, different priorities, different norms. Crimea, Basque, Scot, Northern Ireland... The economies are also sufficiently varied that one ring to rule them all is absurd, resulting in tensions between the historical cultural groups, aka countries. The UK is increasingly interesting on this matter.