Money and countries aren't "ephemeral", whatever you mean by that. Countries are large collections of individuals and groups united by common culture, language, geography, economies, religion, etc. They're as real as your family or friends group is, because they're based on voluntary association.

Money is "real" because governments accept it for payment of taxes. Taxes are not optional, thus businesses find it convenient to ask for payment in local currency. This creates a common medium of exchange as long as people have faith that the current government will survive.

So no, they don't just have value because "we decided" they do. We can't change our minds tomorrow and shift all the countries and money around because they are based on reality.