How do dragons collect that much gold?
Like, even if you had an existing pile of gold you wanted to move to the cave, and you had backhoes and dump trucks, it would take a lot of time to move that much gold. But imagine if the gold wasn't already in a convenient pile, and it was scattered around the countryside. That's a lot of sifting! And then you have to carry it like a claw-ful at a time??
Plus, gold is very heavy. Difficult to fly while carrying much of it!