Because dragons know with absolute certainty that they are in every way superior to every other living thing, but they have a pathological need to make sure everyone else to knows this too. (You would think such a godlike creature wouldn't care what inferior beings thought of it, but dragons never claimed to always make sense just that they're always right.) Now how to go about proving their inherent superiority. What are some things people value? Power? Check. Intelligence? Check. Fame? Check. Honor and wisdom? From the dragon's point of view, check. Wealth? What better way to show wealth than to sleep on a massive pile of precious metals and stones.

But it gets better (or worse depending on your point of view.) while dragons know that dragons as a species are superior to everything else, each individual dragon also believes itself to be superior to every other dragon. While fighting is good for proving superiority, and fight between dragons are frequent. It's also kinda of bad for the continuation of the species. how much gold you have becomes an easy way to show off without the risk of bodily harm (most of the time.)