Many countries have big internal variations within areas, just as much as between other countries - by your logic why is it not just as necessary to have separa

Many countries have big internal variations within areas, just as much as between other countries - by your logic why is it not just as necessary to have separate currencies for California, Southern Italy etc? London, New York, Shanghai, Paris, Singapore would be better served by a common currency than their national ones according to this line of logic, and given that they make up significant proportions of the economy for their respective nations, you'd think that enforcing use of the local currency is hugely damaging for their economies.