I think maybe some allergies are genetic, or maybe they aren't allergies, just reactions to certain foods, and then there are true, life threatening allergies. My brother and I are "allergic" to fresh pineapple and fresh kiwi. I like the taste, but the resulting canker sores aren't worth it. DH and both sons cannot tolerate penicillin. One son cannot tolerate sulfa based antibiotics. We learned that the hard way with a frantic ambulance ride! I have a friend who says she is allergic to "everything", but has never had a reaction to anything! I have another friend that cannot be near certain kinds of seafood, and always has an epi-pen on hand, and I've seen him have to use it.
As to causes I think there are many reasons/causes. Allergy, immune deficiency, environmental, preservatives, chemicals, and a myriad of other factors. You are right though...I don't remember much talk about allergies when we were kids.