Their currency is accepted, or can at least be freely exchanged for euros, but that's not what this is about. Almost all of those natural resources are extracted by western corporations. They leave just enough of that extracted wealth behind in wages and taxes to make World Bank and IMF loans possible for the country in question so that the country can pay for food and other essentials from abroad, generally from the US. The conditions of those loans require more resource extraction by western corporations, and more privatization of every other sector. Developing local agriculture to ensure food security is forbidden, because cash crops for exports are more profitable, and a country that isn't food secure is easier to oppress, especially when the valuable resources have all been extracted and taken to the imperial core.