I'm trying -- and failing -- to figure this one out. Maybe because I don't see it as some magical ability that some people are granted and others aren't, but as a specific set of skills, which to some extent can be picked up by most people?

That is a really good point. In a way sometimes intelligence gets seen the same way, where people want to make it a magical ability that appears in only certain ways. When really social ability is a combination of different things and is definitely something you can learn. Even people who would be considered good at it sometimes stumble if they're in a different social setting where they don't know the rules or the signals are different. But sometimes the people who want to define themselves as incapable of that sort of thing are really saying that they want to be the person who doesn't do it.