The reason for not doing this is they want you to play within your level range. If you could sit there and farm level 30 and then buy level 15 boxes we would have the same situation before where new players are just being destroyed. For obvious reasons that is not a good thing since it will stop growth of the game.
Due to not resetting the already owned items, this idea isn't good. I currently don't have the equipment to play PvP on level 30, but I do have some equipment to play on level 18. However, most of the weapons are common, as I cannot get better ones through boxes. This leaves me with two options: Fight as good as I can, only using fusion to upgrade my stuff or quitting PvP completely.