If you've ever tried playing Spy in TF2 you'll know that players will shoot at anything that moves, and anything else that doesn't. GG2 just has the infinite ammo to facilitate firing everywhere for no reason. Consider yourself lucky that you can't be bumped into, and have guaranteed kills no matter where you hit rather than being required to be behind the target.
Stabbing someone who doesn't notice you works merely because that person wasn't aware of you. You gotta be fast, and you gotta be out of the way until you attack. If someone expects a spy then they will spam in every direction just to make it that little bit harder for you.