I'd say the more clear correlation is social media. Not only can weak-minded people be pulled down various conspiracy rabbit holes that allow them to rationalize their actions, it's also never been easier to become "famous". Given that these shooters are almost always social outcasts craving extreme levels of attention/validation, social media lubricates their fantasies on both ends of the process. Plus the copycat factor from other mass shooters.

That said there's no doubt there's in my mind economic conditions contribute. The paths to middle class prosperity in the US are fewer and further between than they used to be. That deprives many of even the aspiration of a better life, leading to nihilism, which can be witnessed in many contexts outside mass shootings (what media is popular, sentiment polls, etc).

I also wonder if our Architecture itself contributes. A lot of bedroom community suburbs seem designed to create social isolation. They're just collections of crash pads for long commutes with no communal gathering places.