It’s an interesting subject.
Reading ‘The Rise and Fall of the Roman Empire” (Gibbon) the author said that incessant warfare meant armies took as prisoner, whole populations of conquered people.
They were kept, traded or sold as slaves. Slaves were currency and if you had or traded any slaves, you had wealth.
Also, the total of gold, silver and precious stones in the known world was limited so there was not enough of that for the valuation of commodities.
Human slaves BECAME currency.
I’m sure that’s part of it....I mean how whole countries became subject to tyranny
except for Gaul where they pretty much wiped them out to the last man, woman and child
Actually, 'Decline and Fall."
He started the story at what he thought was the peak of the Empire, the 2nd century.