Because currency is mostly machines, not a lot of human labor there, and resources are natural. No labor there at all. A product is made by human labor to create an object, or a thing that people will buy and use.
A thing produced by labor would be considered as a product.
Currency is an item or multiple items that you would trade for something else (a set amount of items for a product or an object) A resource is usually an object that creates a product.
