Because in the end with a fiat money whether from a private bank or central bank gets inflated till the money is worthless..... every time. The temptation and potential profit is too great both for businesses and politicians who want to spend but doesn't want to tax. Gold puts a limit on what they can do, it stops the warfae welfare state and keeps our wealth from being diluted through natural supply and demand and not a small group of people that includes congress.