In D&D (typically) Dragons stay in remote places or entirely different planes. The odds that a party of adventurers would ever end up fighting one that isn't an adolescent trying to assert it's dominance is pretty low. Ancient dragons, even evil ones, are usually wise enough to avoid conflict that would just get an even bigger fish to come out. That's why ancient black dragons aren't popping up to destroy the world as in some instances they so easily could, or they're just in hibernation. The world tends to have a certain balance to the forces in it, and the bigger players stay out of the way or they'll have to face each other. Gold dragons don't fight evil, Black dragons don't fight the good. If one comes out, the other will come out for balance, so it's better if none come at all.