Because these are the skills that you use to attack and defend in a social conflict! Social conflicts are outlined on YS 215, after the rules for physical conflicts. Social conflicts use Empathy to determine initiative, rather than Alertness (which is a big sign, from my perspective, that social conflicts are meant to be an entirely different method of conflict resolution). A character with high social skills will be far from useless in a fight, as social skills can be used to maneuver, but really comes into his own in a social conflict, where his social skills can be used to directly attack the enemy.
But why have them if they're so completely separate? Again, it becomes that bone you through your noble before the inevitable brawl with the equally inevitable thumb twiddling and cringing. Now, don't get me wrong. That's hyperbole. But it's valid hyperbole that services a point. Either social attacks are viable attacks that are integrated into the game play, which means it's silly that "shamed" can take the place of "broken ribs", or social attacks are to be completely relegated to a separate arena, which if that's the case, it drastically lowers its value as being part of the world as a whole. It becomes a mini-game. It's kind of like an rpg "separate but equal" argument and falls apart for similar reasons. Again, hyperbole, I'm not making a literal comparison and realize that I'm comparing trivialities and amusements to important stuff, I'm just trying to show a logical similarity, not a moral one, if you take my meaning.