Perhaps your error is thinking of currency as a part of the natural world, rather than what it really is, an invented medium of exchange. In order for it to work in its intended fashion, it must have some scarcity. It can’t be so scarce that it hinders growth, and it can’t be so abundant that it causes too much inflation. It’s not the only means of exchange, it’s simply the most efficient one, and the role of the central bank is primarily to maintain appropriate balance between the forces that push and pull on the money supply.