[s]Because [i]I[/i] am the only god! Mwahhaha! Bwaahaha! MWAHAHAHAHHAAAA![/s]

Actually, Arceus is not included because it is overpowered. Dawnstar is right. Mew, in this roleplay, is more like a Adam/Eve god. Arceus is considered just as powerful as the Judeo-Christian-Islamic God, in that it literally could fix the world whenever it wanted (or kill everything), and there'd be no point in strengthening all our legendaries and slowly healing the world.