Well, most of the treasures were put in one place -- the planet Crysilis, which used to be empty and boring before it was filled with gold and silver. However, Crysilis was full after a while, so Devan exported the various treasures to other planets - I doubt he really expected Jazz to come along and resteal them. Added to that, there's the treasures which simply come from the planet, like the fruit in Jungrock, instead of being exported. As for the weapons, they are probably for use by the turtles and creatures occupying the various planets - there's got to be some intelligent life out there somewhere that would resist conquest.