That's because the people that the men with guns can use them on have very few real assets (both physical assets and human capital), so they can't give up very

That's because the people that the men with guns can use them on have very few real assets (both physical assets and human capital), so they can't give up very much in real value in exchange for the currency that the men with guns are demand. That leads to the currency not being very valuable. In places where the people do have significant assets that they can give up in exchange for the currency, the currency has a lot of value.