actually they did it in 2 as well sometimes

Only if you don't have assault rifle training, IIRC. Then you'd pull an Avenger out and use that, since everyone apparently knows how to use it. But if you had an assault rifle, pistol, or SMG, you'd use whichever you had when appropriate.

ME3 was really weird when it would and wouldn't use the appropriate weapon. Sometimes I'd look all cool with an N7 rifle, and next scene I was using an Avegner for some reason