I think that the reason why melee (or indeed any close range combat) is so finicky in fps´s is that aiming at a nearby target is harder in games, enemies can survive repeated hits, move unnaturally (sprint, jump in the air, go prone midair, turn 180 degrees, land on concrete, all while reloading a machinegun...). But the biggest thing is that in most if not all online shooters, the enemies don´t react to being hit (sure, being stabbed probably does not result in an instant kill but the victim probably won´t just ignore it like nothing´s happened)