I think (and there are studies to back this too) that the more you eschew an allergen the more violent your reaction against it would be. I am from a third-world country and I know no-one from my country that has allergies related to foods like peanuts, sesame or gluten. The only other explanations for this would be: a) These allergies occur so rarely that I haven't heard from the minority of the people. b) Or, the people who had this allergies have already died without diagnosis. c) Or, people have these allergy but choose to hide them for fear of social shame. d) Or, people don't get medical checkup that allows the discovery of these allergies.

But I think the case that people being accustomed to the allergens due to forceful conditions is the best explanation for this.