Might have something to do with the "American Dream" not happening for these shooters.
I think it has more to do with lack of a quality screening process.
Even when comparing ourselves to other first world countries with more permissive gun laws (though, I know the list is fairly short), we're still the standout when it comes to the frequency for which this occurs.
If you compare the US with the Czech Republic, it's an eye opener.
They, like us, allow ownership of handguns, shotguns, semi-auto rifles, etc...
They, like us, have "shall-issue" concealed carry permits.
They, like us, have roughly 3% of the population having said CCW permits
Interesting additional notes
They actually have a more permissive CCW system...they don't have gun free zones (you can carry in your kid's school if you're picking them up) and they have universal reciprocity among their administrative districts as where in the US, there are some states that you can't go to with your home state's permit.
What they do have, that we don't, is a thorough vetting process that acts as an "upstream check" before someone reaches "gun owner" status that involves extensive training, education, and clean bill of mental health from their physician.
The outcome is, a nation where you can pack heat, but that has only 1/5 the homicide rate (per capita) that we do, and a fraction of a percent of the mass shooting events.
Screening works when done properly. Even in the states that do enforce background checks, the FBI/NICS background check process is weak. There's no mandate for training required, there's no test to pass, and the only time a mental illness disqualifies you is if you've ever been involuntarily committed to an institution by a judge via court order.
Theoretically, a person with bipolar disorder can easily pass the background check in the majority of cases.
