I would imagine because playing a character who CAN destroy the world and a character who WANTS TO destroy the world are two very different experiences. (The "everyone knows you can destroy the world" is a fantastic touch, by the way.)Then develop the character. You want a PC who can "end the world", make a wizard who quests to create a world-ending artifact or something.
Why "give" such a PC to a player? Make them work for it.
If you just want to "give" it to them as you did, you don't need a class or subclass for it. You make it part of the story that your god has given you this horrible power and responsibility or something. Now you have what you want and you can have it with ANY class and race.
I agree it does seem tangential to the class/subclass discussion, as I don't think that would ever be a published concept. But I do think running it as a basic fighter with a plot hook would cheapen the idea, somewhat.