Reason one is modifiers. If somebody tries to do something unreasonable, say "I want to convince the King to hand me over his Crown and Kingdom", or "Hey, I want Jerry's PC to suck my cock", with no leadup or pretext whatsoever, well that's what modifiers are for. You have to be really damn convincing to make somebody actually kill himself with just a short talk. It's no different from any kinda outlandish idea like "Hey, I want to lift that bus full of sumo ringers with my bare hands!" Sure, you can try, but have fun actually making that roll. Social Conflict is no different and still subject to modifiers.
The second reason is, that the players and the DM still play the character. So, if you are diplomized, convinced or whatever, YOU still play the reaction. You should follow the spirit of the roll and try your best to roleplay your character in a way, that shows the result of those rolls. And if somebody tries to weasel around and twist himself out of a successful roll, well, that's what the GM is there for.
Now, all of that just goes for basic Social Conflict rules. Skills for Bluffing, Convincing, maybe one for logic and one for pure emotions, skills do defend, whatever. Most things can and should be left up to the GM, the players and the situation. But that's not really "Social Combat Rules", or at least not what most people mean when they talk about it.
Social Combat rules, stuff with "Social HP", moves, feints, multiple rolls, essentially full fledged combat but with words is pretty much unneccessary. It would be easy to write, yeah, but it's unneccessary.
What's the point of combat rules? It's to simulate something that can hardly be done at the table AND it's there to provide gameplay and fun. Could you distil combat down to one or two rolles? Certainly, but to most people, playing out a fight is fun, it's there to provide contrast to the roleplaying and adventuring.