I suspect a part of this is due to America's losing of a Spa culture. Korea, Russia, Germany and Scandinavian countries seem to have (comparatively) thriving cultures, which make people more comfortable being naked around each other.

I do agree with the common sentiment here though, what a tired, boring trope to pillory millennials for this quirk. It's hardly ideal that they're becoming more body conscious, but it's quite a great solace to instead appreciate how much they're advocating for tolerance in our society.