Hypothesis: The muggles don't possess much gold. Most of the huge stacks of gold in places like Fort Knox are clever magical replicas, and have been for a very long time. Any wizard can easily see through the ruse, but the muggles are clueless.

How do we have gold that we use as a conductor? Perhaps when a muggle handles fake gold, it gets magically swapped with real gold from a small supply elsewhere. Or else, maybe fake magic gold is a really good conductor.