Because if the elves unify the world, the first target for external invaders is definitely the World Tree. Only by destroying the elves' World Tree can they obliterate the elves' war potential.
If they only consume at the periphery, external invaders without logistical support are impossible to win against a forest that is continuously supplied with living forces based on the World Tree.
Since the opponents will inevitably target the World Tree, there's little difference in whether these Plane Keystones are hidden or not.
Rose's face was full of pride and arrogance, saying: "With the Empress's strategic foresight, the world can be commanded and settled with a gesture! We elves, possessing the World Tree Root, can, despite any foreign invasion or monsters, deploy the Imperial Court Guard to any location at the first opportunity to pacify them. The world should belong to the forest, much like it was at the dawn of time, when life had yet to exist."
