School shootings weren't always normal. Prior to Columbine, which emerged from the catacombs of the ancient internet, these kinds of things happened, but not with the frequency or force they do now. Of course, the massive amount of publicity lumped on the two Columbine boys didn't help, virtually assuring they would be enshrined as antichrist-like heroes. Perhaps this would've happened regardless of media attention, but whatever the case, I don't think it'd be far off to say that the crisis of school shootings in America began here.
I don't like him any more than you do, but Michael Moore in his movie Bowling for Columbine speculated that the normalization of mass violence through foreign wars abroad contributed, in some sense, to its citizens' predilection for it. On the day of the Columbine shooting, Bill Clinton authorized the largest single-day bombing of Yugoslavia during the Kosovo War. Not saying it's the perfect explanation, but at least it at least goes some of the way to getting at what's behind the violence instead of impotently crowing about guns.
If the assassination of Martin Luther King signaled a shift in American politics from the practical business of bettering lives to the zero-sum psychic warfare we now know it to be, Columbine indicated the complete abdication of government from bettering lives. Sandy Hook took this a step further, ensuring that we'd never address fundamental questions of politics again, and instead, spend our time bickering over "gun laws" as if they were a magic talisman one could wave in the air to make all the evil go away. What would in a healthy society provoke introspection and course correction now is useful only for partisan vanity points and moral indictments of political enemies.
People's reaction to these incidents — the immediate turn towards stumping for legislation and partisan politics — is almost as disturbing as the act itself. The country has stopped asking important questions and instead bickers over laws and reforms that would reduce the number of deaths or something. Presumably, the mind virus of dehumanized, top-down, technocratic rule has leeched outward into the minds of voters who see a casualty number and their first response is, "Imagine how much lower that number could be if..."
The question we should be asking is: what kind of political system produces these results? If your answer is guns, you're answering the how. We're not here to discuss the how which is highly variable (knives, cars, bombs, etc.) If you want to discuss that kind of thing, there are plenty of media outlets and politicians like CNN and Bernie Sanders available to you that will discuss this ad infinitum.
Sandy Hook should've been a come to Jesus moment, but instead, gun control advocates papered over the Dark Night of the Soul with laws and legalese, ignoring the very real spiritual and cultural void left gaping by the events. Could this very approach — impersonal, legalistic, by the numbers — be the thing that has produced these incidents? Could the cold, nihilistic society that such an approach to politics yields be damaging to its people? Is the reduction of a person to his gender, race, and sexual orientation — increasingly, the only metric available to spreadsheet leaders for how to judge people — alienating more and more people?
School shootings are explicitly political events and would be treated as such in a nation seriously concerned with how its politics affected its citizen. In its highest form, politics is concerned with the betterment of its citizens and their protection from enemies and infringing forces, be they physical or spiritual. An attack on a school, one of the more sanctified institutions erected by the modern political body, is to strike at the marrow of political order.
Our leaders, presumably, are not concerned with these kinds of things. Biden took to the stump yesterday and barked like a good dog about "standing up to the gun lobby" implying he'd be satisfied with a government that yielded school shootings with only five dead instead of twenty. That people commit these acts of brutal violence is not of concern to him. In other words, the chief end of politics, to better the lives of its citizens, has been abandoned wholesale and replaced by the nihilistic jostling for partisan one-upmanship points.
Go to any drooling liberal rag or YouTube streamer and you'll get sanctioned talking points about gun regulation without any self-reflection on the basic premise of these questions. Maybe the entire viewpoint — a reductionist, if only he had a pistol instead of an AR so the numbers could've been lower, veering towards the trolley problem ethical arbitrage of "we would've been happy if just his grandmother had died" arena — does more to perpetuate them than to prevent them.
So, what's changed since Columbine?
It hasn't been the guns.