Because the majority of people are idiots who automatically believe what the media feeds them and what people tell them.

The media tells people that people shoot up schools because of their mental health issues and people automatically think those with mental health issues are capable of shooting up schools. Society tells people that anybody who is different and cannot or does not want to conform is a threat, and is therefore, "crazy". Many people are incapable of critical thinking because it's easier to automatically assume society, the media, or the government are knowledgeable and truthful.

So I blame the ignorant masses, or what I like to call "sheeple" for all the stigma. Yet, ironically, many of the worst cases of people doing messed up stuff like shooting schools, rape, or murder even, stem from people not willing to accept and help people who are struggling with mental illness.

I mean, one would be surprised as to how something like showing somebody love and compassion instead of treating them like garbage can change a so called "evil" person. Many people become bad because nobody shows them love or any kind of empathy in their life.

To make a long story short, I blame the masses for everything.