Maybe it takes more energy because of the diversity of materials? If we used an order of magnitude less types of plastic than we currently do, it might become more economical because it wouldnt require as much mechanical labor to sort and process. Removing shipping from the equation seems like the removal of quite a bit of carbon.
When you melt plastic down, you get lower quality material than what you fed in. It's not a closed loop. Plus, the wide variety of available properties are what makes plastic so useful in the first place.
