I read a paper a while back that kids who are exposed early to more things (dirt, pollen etc) tend to have fewer cases of allergies later on in life. I wonder if that’s partially what’s going on in this case. (not the same link, but it talks about it a bit here) https://www.livescience.com/health/allergies/is-playing-in-the-dirt-good-for-kids-immune-systems