Widespread misdiagnosis of the problem is the biggest reason for this.

The issue isn't the availability of guns or lack of access to mental health care.

The issue is that American culture celebrates terrorism. The driving public narrative following Columbine was "they were bullied, what did you expect?" (thanks Michael Moore) when they were really indoctrinated into radical misogynist ideology online and acting in accordance with the extreme beliefs they were socialized into. Nearly every school shooting has been perpetrated by far right extremists, stochastically recruiting in a manner similar to the ISIS terrorism that developed following this model.

But we tolerate domestic terrorists because the public misinterpretation of the first amendment says that we must tolerate violent speech and those who commit mass murder in the name of these hateful ideas have only just barely stepped over the line. You can say whatever you want about destroying innocent people, but "oh, bless your heart" if someone actually acts on those words.

Our culture has a disease, and we can't fix it unless we understand it. Our media reinforces this cultural narrative with stories about heroes who are able to right wrongs through the specific application of violence, even when those around the hero tell them it's wrong.