Genuine Question

Aren't True Dragons referred to as a supreme spirit because they are the creator god Masterpiece, help build the foundation of the world, and are above the Grand spirits in Existence My question is how the literal fuck Zegion became a supreme Spirit, Did I miss something, If he can become one just like that, what's the hell is stopping Milim from becoming one or Guy Crimson or Even Feldway. To me, this doesn't make sense, so if someone can explain the logic here I would appreciate it