A few of the options are close enough:
1. Good-aligned and Lawful Neutral nations have no trade with monsters. That leaves
3a. A great many people, in the wider sense - especially humans and halflings, but including neutral to evil individuals of every race - will trade with anyone who is even marginally more likely to pay for their goods than to kill for them, and
3b. Some nations positively like to trade with monsters - the evil nations - and gold is most agreeable to them. Further,
4. Dragons hoard gold for reasons of their own that have nothing to do with giving it away again to puny humans.
1. Good-aligned and Lawful Neutral nations have no trade with monsters. That leaves
3a. A great many people, in the wider sense - especially humans and halflings, but including neutral to evil individuals of every race - will trade with anyone who is even marginally more likely to pay for their goods than to kill for them, and
3b. Some nations positively like to trade with monsters - the evil nations - and gold is most agreeable to them. Further,
4. Dragons hoard gold for reasons of their own that have nothing to do with giving it away again to puny humans.