The twins were a bit sad that they couldn't get to eat Dragons, thus Lin Mu had to explain to them why they couldn't eat them.
The biggest limitation other than their obvious strength was that most dragons were actually forbidden to be eaten by the Immortal Court as well as the Temple of the Immortal Court.
While the Dragon Clans weren't exactly united, several of them still had close alliances with the Azure Dragon Clan and thus had immunity and a special status.
Which only left the lower Dragon species that weren't considered 'noble' to be free from the restraints and could be eaten. But even then those were still very strong with the weakest one being a Seventh Tribulation Stage Immortal.
If a Dragon appeared in a world, it would usually become one of its supreme powers.
