Self defense = Awareness
I think it is because they instinctively know "you just don't hurt people"- unless they are a damaged aggressor who doesn't have that internal regulator.
So, normal people have a fear of striking too soon or striking and hurting the wrong person. They worry about getting in trouble for hurting someone. They don't want to attract attention. They wonder if they are misinterpreting the situation- or instinctively assume the person approaching won't really hurt them. Even in obviously dangerous situations, such as the "9-11" planes, this hesitation and self-imposed reluctance to hurt a bad guy will prevent action from being taken until it is too late.
Sadly, this is because normal people are nice. The bad guys out there are not and have none of that concern. To them you are less than human. You are prey.
I don't know of any healthy way to turn a nice, normal person into a person who can switch that off and strike an aggressor fast and hard without doing some other damage to the psyche, too. Some self defense training probably helps, but I think even this does some damage in most cases. It isn't healthy to see the whole world as a threat, because that isn't the reality of the situation.
And, split-second judgments are always iffy. In most cases, people are just not aware of a threat in the area until it is too late. They don't notice their surroundings at all.
The best line to walk between being paranoid and being prey seems to me to be awareness. Pay attention to your surroundings at all times. Observe. Not just to see threats, but to see beauty. Observing the world has a big payoff in every aspect of your life. It's the difference between being conscious and aware or being asleep and shuffling through life seeing nothing but the ground in front of your feet.
And then, when a real threat appears, noticing it early may just give you time to decide when the right time to strike has come. The predator has already noticed you- even the playing field.