Publishers are born connectors; they bring like-minded people together. They are also conversationalists of the first order. They foster the interaction between the three key parties in commercial media: the audience, the author/creator and the marketer.
There's a reason publishers don't build on top of social platforms: publishers are an independent lot, and they naturally understand the value of owning your own domain. Publishers don't want to be beholden to the shifting sands of inscrutable platform policies.