I live in Eastern Europe and not one singular person I know has allergies.
Americans seems to have a sensibility to everything, gluten, peanuts, all types of foods and substances.
Why is that?
There is not a perfect answer, but one theory is the “hygiene hypothesis.”
The world kids grow up in is too clean, and the immune system is not exposed to enough things early in life. The immune system struggles with what is helpful/harmful when people are exposed to new things when they are older.
There’s a big difference between being allergic and having a preference. Most of those who say they’re gluten free, for example, don’t actually have celiac disease.
Allergies are an immune overreaction. Something gets in your body that is harmless but your body doesn’t know it’s harmless, and goes completely scorched earth trying to get rid of it.
It’s like if the US were to start punching in nuclear codes because someone left a somewhat suspicious package at a Walmart in North Dakota.
For this to happen, your body needs to be unaware that something poses a risk. If you spend your childhood outside, romping and playing in the dirt, your body’s gonna be exposed to a bunch of stuff, and it’s gonna know what can and can’t hurt it.
However, if you spend your childhood mostly indoors, constantly in a hygienic area, your body won’t learn the difference for some things.
Some, but not all, American children spend most of their time indoors and hygienically sheltered. Allergies develop in these kids because their immune systems don’t get exposes to some common things.
Tl;dr we don’t rub enough dirt in.
Sensitivities are different than allergies and cause different reactions.
The eli5 answer is that medical science does not know yet, but Australia and the UK have very high rates of allergies as well compared to other countries that consume a lot of peanuts, like Israel.