I've never been to the Mall of America, but I've been to similar if smaller things in Southern California and elsewhere. It's true that such projects can have difficulty with land assembly and the financing of infrastructure, but it's also true that they can solve certain coordination problems and internalize some kinds of externalities that "real" cities are unable to. And the "private property" aspect of them means they can do law enforcement (security) in a way that real cities can't actually do (throwing people out if you don't like them, basically).
These aren't my favorite places in the world, but they're fine for what they are. For me the mystery was that (though this has been changing) they have been envisioned without any kind of residential component. Or, more specifically, why not make them a bit more like real cities, with actual nearby residents.
