Because the quantity of dollars is 50 times bigger than the quantity of gold. For example if the total gold in the world is 50.000.000 ounces and the total dollars are 2.500.000.000 then 1 ounce of gold = total dollars/total gold = 50 dollars.
There are actually five billion US dollars in the world, not counting the 1340 billion dollars in U.S. federal reserve notes. And there are 3,8 billion ounces of gold in the world. So the price should be 365 dollars per ounce of gold. But I guess other countries money supply should be included too, since we are talking about the entire worlds gold then we should also include the worlds total supply of money. Which makes gold worth a lot more...