Because when the value of the dollar drops, people tend to invest in gold. This causes an increase in the demand for gold. This causes an increase in the price of gold. It's not a direct relation to the value of the dollar...It's a relation to an event that causes the demand to increase.
It's like my gas analogy. Gas doesn't cost any more to produce in the summer as it does in the winter, so why does the price go up in May/June? Because the oil companies know that demand will increase in the summer.