[ZenGum]

Almost certainly, there are many factors involved.

The relative availability of guns, especially handguns and high capacity rifles, in the US is almost certainly one.
But there is also the attitude towards them, as already mentioned. Switzerland has lots of guns, but they're regarded as mundane, kept at home. In the US, it approaches a fetish or even a religion. "Guns keep us safe from tyranny!" etc. To the mentally ill, the idea that Guns Are The answer is easy to absorb from the pro-gun statements regularly and loudly made.

The media's fascination with these events is another factor. For the mentally ill, the saturation coverage of events like these soaks in and makes it seem like a normal or reasonable act. I wish I had saved it to share here, but I recently saw an infographic of the shooters of the last dozen or so mass shootings. Each shooter had a little box with name, date, types and legality of weapons and number of victims. They looked like baseball cards. The shooters were listed, not by name or date, but by number of kills.
What better to motivate some insignificant-feeling psycho to try to top the list?

And yes, culture. Think of all the wild west folklore, the western movies, where one or a few "good guys" with guns defy and defeat a bunch of crooked land barons/ rustlers / outlaws / corrupt officials etc. There are so many cases where this happens. After a few hundred such movies, the idea is unconsciously absorbed that, if you're repressed and downtrodden and the system won't help or is in fact the problem itself, armed defiance is the correct response.

There is no single factor that is "the" explanation, and so there is no single action that will fix the problem.