Most of the non-terrorism mass shootings of the past two decades I'd say are a result of psychopharmaceuticals (antidepressants, antipsychotics), the internet, and the media - or rather, a perfect storm combination of the three. The side effects of legally prescribed drugs are often worse than the diseases, turning people into emotionless zombies, and in other ways exacerbating mental health problems rather than effectively treating them.
The internet acts in multiple ways, to feed into people's depression and psychoses, but also to act as a medium for expression for those who want to go out perceived "glory", or propagate a manifesto. Traditional media also does the same, overcovering every single mass shooting as if there aren't hundreds of thousands of people dying elsewhere. News should be covered, but they linger and obsess, potentially inspiring others to "go out" the same way.
You won't see blame put on prescription drugs because most of the media runs on prescription pill ad money (turn on CNN or Fox and watch a few minutes) and without strong evidence - and let's face it, it's just easier to blame "crazy" or guns - why take a chance legally or financially?
It's not a coincidence that the school shootings in particular started escalating in frequency right around the time drugs like Prozac came on the market (and many, many other drugs since then, leading to overmedication of the population as a whole) and were overprescribed to kids around the country. Sure, correlation does not imply causation, but I think the link is stronger there than it is for just the availability of "military-style assault rifles", or some inexplicable increase in crazy people - we've always had crazy, what changed? - which are the typical reasons sited by those from the left-right paradigm.
You can trade a chicken for an AK-74 in many parts of the world, yet they don't see the random mass killings that we do here. Yes, you see terrorist attacks, ethnic cleansings, criminal conflict, etc which have always occurred in some form, but not the random person "snapping" and buying a gun to go on a killing spree - or at least not at the same level outside of the Western world. No one thinks to say maybe we've got some other problem going on here?
I'm not a conspiracy theorist. In fact, I don't think it's a conspiracy at all so much as people just turning a blind eye, the same way we've tried to do with the opioid epidemic, or in decades past with the tobacco-cancer link. Or how mental health used to be viewed in the early and mid 20th century (lobotomies, electroshock therapy) before reforms took place, and books and movies like One Flew Over the Cuckoo's Nest, and Girl, Interrupted enlightened the broader public. No one wants to talk about how depressed and consequently medicated we are as a society. It's (ironically) depressing and bad for business.
P.S. I do not give a rat's ass about gun control one way or another for the sake of this argument. You want to limit guns or even ban all guns? Fine. If an authoritarian regime comes to power and revolution is necessary, we can always steal their weapons or make our own, worst case scenario. You want unrestricted sale of guns or just keep the status quo? Fine. I'm not cowering in fear because guns are everywhere, though personally I don't feel compelled to buy a gun. Whatever. I just hate that the root problem gets ignored.
P.P.S. I do have a bow and a sword, though. You come for mah weapons and you will lose your head (there can be only one) or take an arrow to the knee.