To be fair it is something of a new phenomenon. Prior to that most people either just died, had parasites that managed their immune system better (places we take anti parasite medicines to tend to develop allergies soon after) or grew up without that clean city living (there is a reason few that grow up on farms have such things).
I guess that explains a lot. The last time I saw a farm, or really any decent plot of field was on a field trip when I was like, 9. Concrete jungle...not for the win, I suppose.