Because money in modern economies is a social and political tool, not a commodity.

It’s pretty difficult to get people in the same nation to agree on social and economic policy, it’s literally impossible to get all nations to agree on social and economic policy.

See, the problem is, most modern economies can create currency simply by crediting electronic accounts or printing currency (though most are credited electronically). The decision on how much money to create and how much to remove (via taxes) is a political decision. People are much more tolerant of decisions like these being made at a national level where people are joined by feelings of belonging to the same group. As the EEU (European Economic Union) has shown, when several nations operate under a single currency, political strife arises when one nation feels other nations are benefiting unfairly. This can lead to turmoil or larger nations with more influence dominating smaller nations with less influence.