But the ancients had good reasons for making their coins of metal. Metals are valuable. Typically coins of higher value are made of metal of higher value.
Metals are durable.
Metals are easy and cheap to form and stamp. In this way, it becomes easier to distinguish real currency from counterfeits. Metal is by no means the only thing we could use. Nobody buries bags full of paper money in the back yard, but people have done that with coins throughout history. If you read about how the new US dollar was designed , you will find that this was a major constraint.