One explanation given for the high number of mass shootings in the USA, compared to other rich countries, is that the USA is an unusually violent country.
Compared to European countries, we have much higher rates of homicides and violent crime, combined with a much greater access to lethal weapons. So it is not surprising we have more mass shootings.

But virtually no mass shooters in Latin America
But what about Latin America? On average, Latin American countries have much more crime and more fatal shootings than the USA does. Yet mass shootings are virtually unknown.
Paul Hirschfield, writing in Foreign Affairs, noted that in the Philippines, guns are sold openly in shopping malls and gun violence is endemic. The gun homicide rate in 2018 was 50 percent higher than in the USA. Yet mass shootings are rare.
He pointed out that countries such as Brazil, Colombia, Mexico and Venezuela have gun homicide rates far exceeding the USA’s. But the Latin American region, which has 2.5 times as many people as the United States, has had only nine known lone-gunman style mass shootings since 1998. Why?
One possible explanation, he wrote, is that the kind of suicidal loners who become mass shooters in the USA have different outlets in Latin America. They may work out their rages by working as hit men or for police, military, terrorist or criminal organizations.
But he thinks the real answer is culture. Extended family ties play a far greater role in Latin America than in Europe and North America. Well-off Latin Americans on average live in larger households, have family nearby and usually live with their parents until marriage.
This way of life promotes values such as loyalty, solidarity and interdependence that help counter-balance individualist values. People who feel stigmatized or victimized are more likely to be defended by their kinfolk.
Of course not all Latin Americans enjoy the protection of extended families. Hirschfield noted that Brazil’s infamous school shooter, who killed 12 children in a Rio de Janeiro school in 2011, had been adopted and lived alone.
But Latin Americans are notable for the ability of unrelated individuals to form voluntary associations and join together for mutual support. This is called “relational mobility.” Levels of relational mobility are above average among US Americans, but the level is twice as high in Mexico.
Hirschfield said that multiple studies have demonstrated that in a variety of situations, Latin Americans are more likely to display socially engaging emotions such as empathy, warmth, trust, and affection, and less likely to express socially disengaging emotions, such as pride and anger, than their counterparts in Europe and the United States. So Latin Americans in crisis may have more moral support available than US Americans do.
My own take on this is that Latin Americans on average may be just as violent as we US Americans, or maybe more so, but they are much less suicidal. Mass shootings are forms of homicide as well as suicide.