Is this just your feeling, that western countries have just as many parasites as developing countries?

That makes me think you don't have much experience in developing nations. Sanitation is a serious problem in large parts of the 3rd world. Availability of clean water is a problem. Sewage capture and treatment is a big problem. These are not big problems in most of the western world.

I'm sure there are studies finding that most people regardless of where they live have some parasites. But the question is whether you have large amounts of the kind of parasites that make you sick and more specifically, the kinds of parasites that can be treated with ivermectin.