From what I can see after a quick Google search, there are roughly 120 guns per 100 people in the US, compared to roughly 7 per 100 people in Israel and 28 per 100 in Switzerland.
Sure, culture may be an issue, and so is mental illness (in multiple cases, but not all). The disgusting political rhetoric that is now commonplace in the US plays a big role too.
But availability of and access to guns is THE biggest differentiator here, if we're comparing to other nations.
It's absolutely stunning to me that so many people, supposedly intelligent and competent, are so unable to implement any change that prevent these deaths of innocent US (and other) citizens. Or unwilling.