It is probably not the sterility of the homes but the lack of outdoor play too.
When Icame in every night from playing I was muddy or bleeding. Actually mostly both.
Kids now spend most of their time indoors playing with those stupid video games and watching cable.
I don’t get much of a reaction to poison Ivy aside from a little in the webbing between my fingers.
My son developed it at about 18 months, so outdoor play (or lack thereof) wasn’t all that relevant in our case.