Why is gold used as currency in fantasy novels?

In order to protect historical records in the Middle Ages, those who write, those who read, and those who look at them have to explode.

They say that because Latin is a base 12 system, the currency was like that.

So in English, the numbers go up to 12 and again it becomes 10+3.

Why do people who speak English use units in the UK and the US?