For a currency, you need an issuing bank. Would you trust someone you cannot elect to issue your currency? The lack of people (politicians and citizens alike) saying yes leads to countries creating their own central bank. For most of history, the world did use one currency - gold. That said, because governments would often debase their currency, you would still have exchange rates I imagine.
Chapter 7, 8, & 9 Curiosities
International Currency
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