Because people see one thing and generalize it. People see a boxing or MMA match, and think if they get hit they'll be bloody and bruised. I've been hit many times, and while I've had a few bruises, most weren't noticeable (except to me) and rarely was there any blood.We don't want to get hit by 100% power. But do we care about to get hit by 30%, 20%, or even 10% power?
But people with no experience don't know that.