As with all these "this X doesn't exist" sites, the images look ok at first sight, but when you start to look closer you notice that some things aren't quite right. In this case, it's mostly roads that don't seem to connect to anything...
That's presumably the same training the AI had :p
Because the problem seems to be in the details. The GAN is great at the global picture but the details are off. And this problem benefits from the fact that a picture of a section of a city should also be recognized as a picture of a city.