Just because the area is there doesn't mean it's finished. And in some of the dlc areas you can tell it's clearly not done, especially the Mars Strike in the Black Garden, with the huge empty box in the back of the area and no boundaries, no enemies, no ghosts, etc. Plus just making new areas out of the blue would require more effort, like choosing how it would fluidly connect to said areas, and redesigning it to fit the area. Most of the time Devs put in the unfinished areas so when you go to download them it's not like a huge download, since it just downloads the necessary textures, updates, CODING, to make it all work. Sure they can just open it right now, but as is, it's just a shell with nothing to do in it. Kings Watch is probably the most developed at this time, as it has the enemies and the ghosts. Of course I could probably be wrong about all of this.