Crime is illegal (we can both agree on that at least
). Therefore it is a particularly brainless criminal who walks across the street towards you brandishing a flick knife, at any time of day. I know that you are not short of brainless criminals in the USA, but they are not relevent here. My point is that true 'attackers', cold blooded premeditators, act on the sly, with ambushes, at night, making sure they both outnumber and outgun the opponent. Self-defense is defense against physical harm, and the majority of the time, that harm will be unexpected.
Think about it - how many murders are the result of messed-up burglaries and gang-bangs, compared with other types of violent murder such as domestic, random, serial, revenge, hits? A minimal amount. Moreover, a gun is useful only in the cases of a known assailant, whom you assess as harmful before they assess you. If a street gang approaches you, and that gang is not equipped with firearms, there are a large number of options to dealing with that. A firearm is a possible solution, but not a practical one, on balance.