Silver and gold do have one advantage: they aren't imaginary. There are lots of valuable substances. Silver and gold have tradition behind them. They're popular because they're popular.
Gold and silver were valuable to ancient people because they are chemically very stable, just rare enough to have value without being impossible to find, and have low enough melt temperatures to be smelted. We have inherited this legacy. Gold was particularly prized in ancient Egypt because it doesn't tarnish. Therefore it worked as a perfect religious symbol of worship for the eternal gods (particularly Ra). The prestige of Egypt made it valuable to the people of the Mediterranean and the near east through contact and trade. Silver was prefered by the Greeks because it is more available there and was easier to carry around than the iron rods it replaced for use in transactions. The expansion of Greek culture led to the widespread use of both silver (Greek influence) and gold (eastern influence) as stamped bullion (I.E. coins). The Far East used mostly bronze as cash coins similar to the way we do with modern coins (this happened in the west some as well). The use of gold and silver in the west survived through the Middle Ages and was only within the last century replaced by a system that primarily uses base metals in coins.
You're talking about metals which weren't even discovered until the late Nineteenth Century, and are extremely expensive to refine.
So does aluminum. One other advantage of gold and silver is that they both can be found naturally in their metallic state ( although it is less likely for silver) and both a fairly resistant to corrosion. This made gold and silver the first metals to be worked and cherished. Copper and tin and iron came next. Copper was a durable metal and available enough that it was useful for tools and weapons, especially once they learned to alloy it with Tin to make bronze which was stronger and more corrosion resistant. Iron was too plentiful in most cases to be valuable enough for coinage, plus they had the problem that with the crude smelting methods they had the metal was very susceptible to corrosion, but its strength made it a natural for weapons and some other uses where durability was important. Most of the other metals were not isolated until the "modern era". And of course gold and silver represented a lot of value in a small easily transported package. A given value in gold or silver would have been easy to carry while the same value in copper might need a cart, while iron might need several.
Hey, they made a whole Age out of copper alloys. Bronze Age? The crude iron smelting methods must have made it to Honda in Japan in the 1970's. Those pups rusted like crazy.
