Value as a currency.
Sure it's the rarity, but also the lack of corrosion. If your coins maintain the same weight for 100 years that's good, because the value can be measured by weight more reliably. Contrast that with iron that degrades, or copper that actually increases in weight as it goes bad... now how do you weigh out a coin?
The not oxidizing and rarity are key. E.g. platinum is also not cheap.
Aluminium is close but it's orders of magnitude more common, and still oxidizes a little.