Why Do Dragons Horde Treasure?

While playing Dragon Age 2 on the PS3 this weekend I killed a few dragons and each of them had treasure for me to plunder. And suddenly it dawned on me. What is the point of this dragon treasure thing? Have you noticed this? In the fantasy genre dragons always have treasure. Why? What are they planning to spend it on? I've never seen a dragon wearing a Rolex or sporting a new Armani suit or driving a Ferrari ... and yet they collect gold and jewels as if their life depends on it. They live in a cave! It's not like they have to buy a house and pay bills. Surely dragons have as little a concept of wealth as the common fly ... it makes no sense. If anyone can justify this convention of the fantasy genre, I'd be happy to hear it.