I don't think you're realizing what happens in several of those instances that you're brushing aside as not violent enough to matter. The results are not what leave people traumatized, it's the process. A gunshot is heard and, with no other information, everybody at the school is told there have been gunshots on campus and to lock down in their rooms and take cover. Students know of the possibility they might be killed, so they start texting their parents and friends like they may never see them again. Teachers start having to make decisions about how much to weigh their own self-preservation versus that of their students, which is a responsibility they are neither trained nor paid enough for. Everybody is trying to imagine who of their friends or colleagues or even acquaintances could have been shot or be the shooter. All of that happens even if only a single person, or even nobody, is actually harmed, and it doesn't just go away once the actual facts come out.
My wife has been in the vicinity of two public shootings at night, when I wasn't there, in the past three years: one at her workplace, one at a Nationals game. The shooting at her workplace turned out to be a conflict between two security guards that didn't extend beyond them, and she wasn't close to any of the violence at the stadium. And yet, I still have nightmares sometimes about her calling me in both cases to say there's a shooter nearby and that she wasn't sure what was next.