These are my current thoughts on why the US has a mass shooting problem not seen elsewhere on the same scale:
1) No real gun control laws
2) A society where wealth, power, education, opportunity are distributed unequally between people of different races, leading to distrust and fear between groups, including various conspiracy theories to explain these differences: Jews control everything behind the scenes, whites invented AIDS in a lab to kill black people, and black people are poor because they're aggressive and primitive.
3) A culture where male masculinity has become like a caricature, completely out of proportion. Owning a powerful, masculine gun is one of the most respected expressions of male masculinity.
In this climate, when a male is deprived of their hopes and aspirations or somehow feels strongly disrespected or humiliated, and they feel they have nothing left to lose, the logical thing to do to restore their honour and hurt masculine identity, is to use that powerful gun to shoot a bunch of people they feel are somehow blameworthy for their predicament.
Think about it, how many mass shooting perpetrators were women?