If you are comparing to the population of the world as a whole, then I suspect that's because many people in poorer regions simply don't have access to therapy.
If you are comparing to wealthy industrialized nations (e.g. Western Europe and Scandinavia) then I suspect that America's lack of work life balance, lack of social safety net, repressive moral culture, high gun violence, etc. play a role.
I suspect that in some cultures people like pastors and family members play some of the role of therapists to a greater degree than in America.