If a good guy is there to prevent a mass shooting, then there's no "news". If a good guy is there to stop a mass shooting then there are already a number of wounded/dead before the stoppage happens - but these incidents tend to make at least local news. But while there are 300 million guns in the USA, there is no guarantee that there is 1 within 100yds of any crime involving a firearm, in the possession of a good guy who is prepared to intervene.
I used to think it was terrible that life was unfair. Then I thought what if life were fair and all of the terrible things that happen came because we really deserved them? Now I take comfort in the general unfairness and hostility of the universe.