>>15574932I like thinking that Arceus was originally far more powerful than its current state, but it poured a lot of its energy into making beings like the Spacetime trio, Groudon/Kyogre/Rayquaza, the Original Dragon, etc. Arceus was powerful but maybe not omnipotent, so it basically split its power among other legendaries to help maintain balance in the world, which is why its current incarnation is only as strong as it is.
Basically, the major, non-manmade legendaries are all fragments of the 'original' Arceus.