This, from Amanda Mull on the Stanley water cup hullabaloo, is basically the very same explanation I would give for the Taylor Swift phenomenon:
Trying to parse why strangers ascribe such meaning to an object or product that is meaningless to you—or why they’re so set on one thing and not another, similar thing—is usually a fool’s errand. Humans by nature turn objects into meaning, and consumerism is the process by which that impulse is commodified by middlemen looking to ascribe that meaning to particular things in order to sell your identity or values or group affinity or sense of community back to you. The product itself, as long as it’s good enough, can be largely incidental to this process. If you look at all of this and see an alienated population and degraded culture, well, I don’t disagree with you. But none of that is unique to the Stanley cup. Precious little is.