I think that this is a very interesting question that I have considered for a long time.
I don't think it's a issue of castigating people (not that anyone here has been doing that), but a question we should all ask ourselves.
Is "self-defense" the same as "self-preservation"? More specically, is the primary reason for wanting to be able to defend ourselves from violent attack, the desire to prolong our lives as long as is in our control?
Put this way, the question sounds ridiculous. Of course, no one wants to be the victim of a violent attack. Even fewer would want their life ENDED by a violent attack. We all seek self-preservation.
But as the original post asked, why do we choose to spend our time learning how to defend against this relatively unlikely threat? Besides health, we could practice defensive driving, swimming, or many other physical activities that are likely to prevent us from dieing a terrible death.
Obviously self-defense is not the only benefit of martial arts, nor the only reason we study. Still, many martial artists I know use efficacy in self-defense as rubrik by which to judge an art, a technique or a training regimen. Even for those of us who are healthy, not overweight, don't smoke, etc, how many of us put as much time and effort into making sure we are not killed by heart disease, breast cancer or in an auto accident, as we do trying to make sure we are not stabbed to death?
I mean that rhetorically, of course. I think that one of the primary benefits of learning self-defense is psychological and spiritual (regardless of whether one studies a "spiritual" martial art). We don't fear death as much as we fear someone exercising POWER over us, and we being POWERLESS to stop them. I believe, without being critical, that for many including myself, martial arts are about a sense of self, about our WILL, and our EGO.
For the martial artist, our training represents how we see ourselves in the world. Some of us may be better than others at defending against forces which threaten our lives, such as cancer and heart disease. All of us are mortal, and will succumb to one of these threats, no matter how much time we spend on health or self-defense. But we train so that we cannot be dominated, so that no other person's malicious WILL can snuff out our own. This is what makes the martial artist difference from the fitness nut. This is why self-defense is not mere self-preservation