i think we need to remember there aren't more than a very few people committing these acts. it isn't a problem with our whole society. every one of these guys as of late has been on some sort of drug .....
I don't remember there being any mass random shootings in the '60's, '70's, 80's or hearing about any going further back in time. It wasn't until the mid '90's that these events (as rare as they are) started happening. That's not decades ago, just about 17 years ago. So, what has changed
Guns? No. Glocks, AR's, large capacity magazines, AK's, and machine guns were widely available to civilians and had been for a long time.
Drugs? Yes. Where only a few people smoked some pot and even fewer took acid or heroin in the '60's, cocaine in the 70's and '80's. Then right around the '90's, you had massive crack epidemic, meth epidemic, high-potencey pot, designer drugs, etc.
Antipsychotic Medication? Yes. Widespread use of these drugs to subdue kids who were "hyperactive" or had ADHD (a "disease" that didn't even exist prior to about 2000).
Violent Video Games? Yes. Starting around 1996, with the introduction of Grand Theft Auto, teens were hit with a barrage of successively realistic and violent, addictive, video games.
Societal breakdown of the family? Epidemic of Fatherless families? This too became epidemic in the 1990's.
A lot has changed in the past 20 years. However, gun designs and access to guns has hardly changed at all.