Dragonkind were the strongest race in the Dragon Plane, and the one whose inheritance had been preserved most completely.
In the memories handed down from ancient times, they knew their ancestors had possessed the power to leave the world, and they dimly recalled that the void held many places fit for their kind to live. But because of the war with the world of Katos, part of those mighty ancestors died fighting in Katos, and the rest died sealing the dimensional rift to protect the Source. Not a single dragon strong enough to transcend the world survived.
After that, with the Source damaged, dragonkind could never again grow to that level. That was also why they had allied with the world of Katos: the existence of the Refinement Art gave them the possibility of continuing to grow stronger.