Many people think they have allergies when in fact their bodies are just reacting normally to something they ate. Heavy farting after eating beans, kale, cabbage or onions is absolutely normal and not an allergic reaction for example.
You're right the "classic" allergies, like hay-fever are often caused by genetic inheritence, but all those new "allergies" have other reasons. A bit hard to read, but i'm referring to this Wikipedia article:
https://en.wikipedia.org/wiki/Hygiene_hypothesis#Overview
»David Strachan proposed that lower incidence of infection in early childhood could be an explanation for the rise in allergic diseases such as asthma and hay fever during the 20th century.«
For some people it is just "fancy" to have an allergy, maybe because they have nothing else to talk about. I have no known allergies, but if i eat beans even the ducks try to say upwind of me…