After Columbine, it's largely became part of the "pop culture", and a semi regular event. Something that makes these often loners get a spotlight and their time to "shine" as all their hate and grievances come out in the open. It's much like folks who turn to suicide as a way to "show them", they want to make a huge scene, go out in a blaze of glory that will make them part of history all while dealing out what they see as some kind of revenge. The media coverage and just how everyone in the country pauses to fight over it, march, discuss, analyze, it's just what they want. Columbine began this trend and others have decided that it's how they plan to go out as well.
It's not the only mass shootings we see, hell we had 500 people shot in Vegas, the worst mass shooting in history that no one can explain even why it happened, and out of that tragedy nothing changed except for a ban on bump stocks.
In most cases since 1999 we've had more laws passed around the US which have granted greater and easier access to firearms. Many states doing all they can to remove barriers and make carrying easier. Sales of weapons also keep going up. States that did have bans and restrictions in most cases have had those bans lifted or expired. Everyone was going nuts with Obama gonna take my guns yet no gun control happened then either. And right now many states are still trying to further pass laws that would make getting access to guns even easier.
After 2020 despite it all, the police were not defunded, in most cases most large cities even raised their police budgets, pay raises, and so on. Yet there has never once been a correlation with size of police force, budget, and crime coming down in response. Some of the most well funded police forces are found in states and cities with highest crime. But again this just shows that the Police don't stop crime typically, in most cases they RESPOND to crime. Often it's too late when they arrive, the crimes are already committed, unless you could post police on every single street corner, they are never gonna stop crime, it's gonna happen no matter how well armed and funded those cops are.