You're mistaking value with use. Gold is easy to work, does not corrupt, oxidize or corrode, and RARE. That's really the answer to your question. Iron and copper are far more useful, but also readily available. Gold has always held a special place in our minds. Religious texts are filled with comparisons to gold's luminosity or incorruptibility. But also: other goals of alchemy included eternal life (elixir of life) and artificial life (humunculous, etc). So not JUST gold. But rocks like lapis lazuli or jade are less useful than granite. But also more beautiful and rare. The point of alchemy was not to get material that was useful (why not just become a lumber jack, then?!). But money. Gold. Value.

I think you are also talking inflation? Indeed, that's why alchemy was made illegal (mostly) and then kings, emperors, and popes secretly had their own alchemists. And in fact inflation due to alchemical gold was a major concern in England in the 16/1700's for instance. When it was thought that the majority of gold currently was just alchemical gold. Which is exactly why it was banned (but Elizabeth, James I, etc all had court alchemists).