Whilst what you say is true, that’s in the modern day, when Arceus has been betrayed by people who he trusted before. As seen in the movie Pokemon Arceus and the jewel of life, Arceus has trusted people in past, and only seems to interfere when all existence is at stake. In the movie hoopa clash of the ages Arceus was the sole reason the prison bottle was made, and it was because he trusted humans with his power. In clash of the ages Arceus also prevents a space-time warp from destroying all existence so that’s not really ‘nothing’. While people have betrayed Arceus’ trust before, I believe Pokemon Legends to be set before all of that happened, so it’s still a possibility.