I used to follow gold prices more closely -- not just gluing my eyes to the screen to watch the price go up & down, but I read lots of articles about the topic.
One common theme was: "If people start taking delivery from Comex -- WATCH OUT!" The assumption was that Comex does NOT have 1 ounce of gold for every paper share that exists. In other words, there could be a RUN ON THE PHYSICAL GOLD if a move ever got started.
It's a close parallel to our banking system -- they print $1 for every 100 digital dollars. If people all decided they wanted "physical dollars" the paper currency would soon run out. There simply isn't enough for all the "us dollars" that have been created electronically.
Same with gold.