It is fragmented by religion and culture. Its periods of political unity have been short. The Congress of Europe held it together from 1815 to 1848. Fifteen European countries came together for a few years before Hitler's rise to power. France, Germany and Italy started the process leading to the present EU in the early 1950's.
Currencies are more than repositories of value for use in market places.
If people have trust in a currency that is a key force binding people together. The German mark has become the common currency in the Balkans yet Germany and the EU have done nothing to pull that conflicted region together. It was Clinton who stopped the Bosnian war in November 1995 at the Dayton Ohio Air Force Base. Much of the strength of the dollar and the yen comes from the two countries' high-tech prowess. Yet while Germany is an industrial giant it is a hi-tech midget.