Because you can just keep piling items in one game, then send them into your new game after you get your first badge and basically make the game have no challenge whatsoever.
Rare items are meant to be rare, e.g. that's why there's only one Masterball per game. As for Eviolites, you can get more in your new game.
Also there has been no generation where you could transfer items since Gen III (I forgot if you could transfer items between Gen I & II).