There are a number of reasons for this:

1. America is a paranoid nation. I've lived in many countries, but the 6 years I spent living in Philadelphia, San Francisco, and Oakland, were very eye-opening. Everyone seemed to be afraid of everything. Outside, they were fearful of being mugged or raped or being the victim of some kind of violence. I've never encountered a first world country where people were so afraid of going around in their own cities. Now some of it was justified, of course. There are more dangerous places in America than I've seen in any other first world nation, but not enough to justify the fear. People fear the police. This one I can understand from observing them in action. People also live in fear of financial ruin. One sickness, one job lost at a bad time, one audit and they lose their house and livelihood. There's no social safety net to speak of in America, so that does make sense I suppose. Americans freak out when men are around children. I experienced this myself many times while in a park alone, where people would even come up to me and ask me to leave because they had children who couldn't play with me "lurking" around. American television was also a very jarring experience. It seemed that their newscasts were to a large part for informing people of the new things they should fear.

2. America is a distrustful nation. Civilians don't trust police and vice versa. Politicians, scientists, leaders... Nobody trusts anybody. And in a nation where judges are elected, this fosters corruption in the systems that support it.

3. America is a divided nation. North vs South is still a very big thing in America. White vs black vs latino is a HUGE thing in America, and the mutual hatred was quite shocking to see. The pure, raw hatred in peoples' eyes whenever I walked into the "wrong" neighborhood or spoke the "wrong" way is something I'll not soon forget.

4. America's political divisions are becoming worse. They where never very good to begin with, as is evidenced in their effectively two-party system. Politics in America is purely war, and compromise has all but disappeared from their lexicon. Nobody is willing to hear the words of "the enemy" anymore, either in office or out. Speaking to liberals and conservatives in the 2010s was a VERY different experience compared to the 80s.

5. America is a vengeful nation. Their tendency to escalate rather than negotiate is especially apparent in their legal, justice, and political systems, and more recently it has become a lot more common in international relations.

6. America is a stratified society. Despite the platitudes of the American Dream, I haven't seen any first world nation with so much social stratification as I saw in America (and also the UK). To make matters worse, many of the strata fall along racial lines, conflating racism with classism, which makes the problem so much more difficult to solve. The sheer contempt of "lower" classes in the USA by friends and co-workers was shocking, to say the least.

7. Bullying is a national sport. I've never seen as much bullying as I observed in America. Their high school groups are very stratified, groups hate other groups, and bullying of people who don't conform is rampant. This continues throughout life, and has been fuel for their reality TV shows in recent years (contrast them with the Japanese ones). American culture tends to favor exclusion.

8. Inequality is getting much, much worse. The ultra rich are taking so much of the American pie and the poor are getting so desperate that it's basically a pot coming to boil. This one thing alone is usually the biggest driver of violence, especially mass violence. This kind of mass violence cannot be prevented when inequality is so high, and it's made all the worse by the ready availability of so many deadly weapons. Weapon violence in America is higher than in any other first world nation.

Basically, America is a pressure cooker not seen in any other first world nation. Mass violence is a result of that pressure.