The thing is, kids have always modified their bikes. When most kids around here get a Walmart bike they remove (or try to) all the decals. Maybe kids back in the day did the same with head badges. Maybe they gave them to their girlfriends, maybe they collected the head badges as they moved on to a new bike.
I've sold a couple separate from the bike as the headbadge could be easily sold and shipped for decent money whereas the bike I would have to deal with locally and get next to nothing for it.