Just look at Borderlands, where the loot levels, it made the game way too easy, by the end of the game you could kill everything including the bosses with a single shot. I think Trion were trying to avoid this. It made borderlands, and also meant as soon as you leveled up all your weapons were terrible until you found one's at your new level again. This system avoids all that.
In this game the level of the item means nothing, it's a rarity that makes a gun different.