Could be all the chemicals in and on our foods. Hormones, insecticides, GMO, etc. The foods we eat today are not the same as the foods we ate in the 1950s. Remember how tiny strawberries and chickens were back then? Now everything is GMO and laced with chemicals and drugs.
What!?! I raised chickens. We had chickens half again the size of chickens now days and strawberries twice the size. Strawberries are picked too early so they have no taste, have too much white and are small.
I do agree about GMOs and other chemicals in our foods and carpets and uphostery, etc. Living 24/7 indoors is not helping kids build up their immunities. Let them play in the sunshine. Let them run around the yard. Let them scrape their knees.