Blame Dave Grossman, retired colonel, author, and fraudulent academic. "Killology" was his invention and he gained fame through the books "on combat" and "on killing". The problem is he went way outside his lane and made claims he has no research to back up. He's usually the poster child for "researcher" who's linked video games with violence, though.
If the fucking media and politicians would stop making the shooters famous and giving big blocks of time to airing their agendas, we'd see a decline. Girls who need fame make sex tapes. Boys go kill some folks. You can be the biggest loser nobody with zero voice and zero fucks given about you, but you go mass murder and suddenly you are famous and everybody wants to know what you believed and why. You think other loser nobodies don't fucking notice that and crave the same attention? Of course they do. In the US you can use a gun, other nations you may have to use fire, a knife, or a vehicle, but it's a global problem because you become globally famous for 15 minutes.
Quit making these assholes folk heroes, watch the decline.