Because that's basically what you had to use for half the game originally. Try using a Shuriken as your main weapon for half the game as an Adept on Insanity before praising anything about Mass Effect 2's weapon system.
I hate when games like Mass Effect 3 use bars to display weapon 'stats', but that's far more than Mass Effect 2 can say where the games tells you nothing. And the weapon descriptions tell you nothing useful about the weapon, just vague descriptors that could mean anything. It was so bad that they had to brute force fixing it by adding DLC weapons that were mostly straight upgrades either for players or squad members. Everything about Mass Effect 2's weapon system is garbage.
