[FredPret]

We can't mine enough gold every year to keep up with the rest of the economy.

To put it another way, if gold was money, there wouldn't be enough new money every year to buy all the stuff we come up in a year.

By decreeing gold to be money and money to be gold, we would be interfering with the market in a major way: artificially giving the value of gold a massive boost and cutting the value of all other things.