Because Deathwing is arrogant and the Alliance is spineless. For too long, the Alliance has seen themselves as the rulers of Azeroth, but the weakness of humans, elves, and gnomes has no place in the world the Shattering has created. [As for Death, that comes back to the Scourge. e considers a moment before he rumbles out the next part, taking Gorehowl slowly off his back]

And Deathwing... is arrogant just as the Lich King was. He will fall just the same.