IT'S NOT! HASN'T BEEN SINCE '71!!
And arguably for Americans citizens (as opposed to foreign governments) not since 1933 when President Franklin D. Roosevelt in a fit of fascist executive orders bans the export of gold, halts the convertibility of dollar bills into gold, orders US citizens to hand in all the gold they possess and establishes a daily price for gold. He then goes on in 1934 to fixe price of gold at $35 per ounce. Considering he had just finished paying people $20 an oz the year before for following his dictatorial orders this was a pretty big time screwing of average Joe. (They liked it an elected him twice more, go figure.)
And arguably not even before the 1933 confiscation as we had a gold exchange standard not a gold standard. It was mostly used for settlements between nations. And if imbalances grew too large, well then, it was poor form to ask for bullion.