Its because they are not really buying the gold on paper anyway, merely betting on the way the price of it will move.
I worked in an arbitrage dealing room for a few years. Most is bought and sold same day, much within minutes. A 1 cent move on $500m is enough to pay the dealers wages for the day
Nowadays, most banks will only allow small amounts of exposure (open positions) in anything other than their home currency at each days end. They learned their lesson in the 1994 crash. I was right in the middle of that, working as a risk management consultant.