How about this, D&D does have some social mechanics, albeit not in the core rules and implemented poorly at best. Such things like Fame/Infamy in UA, Guilds in a variety of books (particularly FR books), and, to some extent, organization affiliations. I believe these are what you're talking about when you mention social mechanics, since these things are mechanics geared to chance how the NPCs and PCs interact with each other dynamically based on the PC's actions.