>>18953855In the 12th Movie, it's explained that the Plates Arceus holds are the source of his Power. Holding all of them, Arceus is a God, but if he is missing some, he can be hurt.
In-Game, Pokemon can only hold one item at a time, and it seems like the Plates In-Game, were scattered across the region, so when Arceus is battled in the games, he is in his most vulnerable state.
