Holy shit way to miss the point entirely. There's nothing alien about these environments. There's nothing to indicate that they are other planets. You just get ported there and get told to kill demons. That is it. You wouldn't know you're on another planet unless the game tells you that you were. A flat textured environment with reused assets a good environment does not make.
Maybe get put into ruins of a long dead civilization that the legion destroyed a long time ago? Maybe a desert planet with a different color pallet? Fuckin' nope. They just reuse the same assets and say "This is a different planet go kill Legion."