Some weapons (like the Atterax) are pretty weak without the right build, but become stupidly powerful once you can capitalize on their strengths.
Most of a weapon's power comes from mods, which you'll collect as you play- a greater capacity & more polarities mean you can fit more and better mods on it. It's true that some are simply better than others, and some start out more developed, but once you put a Orokin Catalyst ('Blue Potato', in the parlance of the game) and 2-3 forma on anything that doesn't have mk.1 in its name, it can handle level 100 enemies.
Your question is based on an incorrect assumption.
