This is such an exclusively American problem.
From time to time, a colleague brings their child to work (because childcare fell through, or something), and they get to interact with strangers! It's weird, they are afraid, they don't know what to say, they generally don't know how to conduct themselves. Oftentimes they are distractingly loud, can't entertain themselves and generally annoy everyone within earshot. And since you are not a parent you are not supposed to discipline the little shits!
Soon they are teenagers and enter the working environment, still without social experience. The Apocalypse is on its way.
and to the OP's point, i'm not a particularly social person, but i love smiling and waving at babies. it's as rewarding to you as it is to the baby. =)
On the other hand if you remove the question of gender I would have to admit that many in the US are more guarded with strangers than people tend to be in other places I've been.