The US is an extremely violent place. A lot of the world is violent, so the US is not alone in that. But Americans have a conception of themselves as living in a safe country, not like all those other countries with people that have backwards regional violence or whatever. And yet the US is exceptionally violent too.

I think what happens is that this discrepancy between the reality of life in the US vs the propaganda and indoctrinated view that Americans have of their culture leads people to doing all sorts of weird mind games with themselves. So one of them is that Americans tend to get desensitized to mass violence and then memory hole it to an astounding extent.

The Vegas shooting was terrible. But it happened years after Americans chose to forget about the Sandy Hook shooting- fewer kids but literal 5 year olds being mass murdered. And of course we could go on and on, the gang violence, police violence, cartel violence, mass shootings every day somewhere, state violence long before the current era with stuff like Waco or the time the cops bombed a neighborhood in Philadelphia, or the bigger stuff like assassinations of politicians, activists, celebrities- there was a period in time where a major leader was being killed every year or so. Americans act like it's banal the number of their presidents that have been shot or had assasination attempts on them. This all before we consider the wars that Americans do all over the globe- more invasions, occupations, coups etc than any other country by far, or the fact that it has the world's largest prison population, etc.

I mean, the US isn't the most violent place in the world or any thing by any stretch, but it's an extremely violent country compared to other rich developed nations and also incredibly violent compared to American conceptions of themselves. You may not know this, but people in other countries are sometimes afraid to go to the US because they think they might get shot. They think of the US the way Americans think of going to the middle east or something.

So yea, when you have this huge contradiction between reality vs propaganda / personal worldview, it leads to massive cognitive dissonance, and one way people handle that is just by pretending reality isn't happening. They just "forget".