A few reasons. For starters, there just isn’t enough gold in circulation with which to benchmark the global economy. Any solution to this, like shifting (i.e; increasing) the fixed price of gold to adjust monetary supply makes a total mockery of the gold standard to begin with.
Also, the gold standard effectively punishes imports. If you have more imports than exports, like has been the case in the US for many decades, it is basically impossible to hold enough gold to balance the sheets. Some would argue this is a good thing since it puts huge brakes on globalization and forces a balance between imports and exports, but that just isn’t possible to maintain in every country. It is also questionable at best to enshrine such balance as the premier principle of international monetary policy
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