There are lots of stories of dragons living in caves. I actually thought the same thing when I first heard about it. Maybe that is some way to mod how frequently dragons appear. That way, each person could set it however they want. If someone, for whatever reason, wants more dragons they can change it or you can lessen the chance of one spawning. Or decreasing the possible spawn points. (I'm still playing with the creation kit so I'm not sure if this is possible. It's just a few ideas.)