You seem to have forgotten your own question.
So please explain to me what does a knife do in a battlefield against RPG launchers?
Katana are used for combat (so it measures against other weaponry) Knife have varied use. Unless you think Katana are practical small tools.
And going for coherence... Knifes are usually caried by soldiers in real war and also something probably "all" soldiers on the game use. The katana? I don't think so.
And about the other points of suspension of disbelief, they are made like that because the gamers want since they wouldn't want to play with a single weapon and be stuck on a trench.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"