Well, there's two reasons for that. First, the vast majority of people who own guns are, shockingly, relatively normal, peaceful, law-abiding, not-the-type-to-kill-without-being-in-fear-of-their-life citizens. Those kinds of people don't shoot at other people because of a messed up reason in their head. Second, the individuals who have shot up schools, concerts, congressman baseball fields, elementary schools, grocery stores, schools, big box stores, movie theaters, more schools, college campuses (shocking, that's a school as well), night clubs, high schools, and more schools... wow, that's a lot of schools... definitely don't have all of their marbles in their bag. Aside from that baseball field with the congressmen, it would take a fair amount of skillful preparation to be able to even attempt an attack on government big boys. That sort of ability resides with the first group, while the will to do so resides in the second.