Kyogre, Palkia, Arceus, etc do to non-legendary Pokemon. Can a weavile, rotom, haxorus, etc take mad hits like specs-kyogre's water spouts or Reshiram's Draco meteor? Haxorus may have more attack that some end game legends but that doesn't necessarily mean that it can compete with some of the Ubers. There are many factors, like speed, defenses, utility, movepools, typings, etc to be taken into account.
As I said, it makes no sense to have weaker end-game legendaries. Arceus sharing stats with Latios has no logical meaning. One is the Creator, the other just a legendary. Who do you think would be more powerful? (Yes, Latios has a higher spatk, but Arceus is still more powerful due to its speed and sheer versatility. )