In the same interview in which Toriyama explains the linearity of Final Fantasy XIII, he also mentioned the lack of towns was due to the fact that in producing HD quality graphics it would take far too much time for the developers to create.
Alongside such comments, Yoshinori Kitase also explains that such a problem relates to being unable to currently consider the continual requests for a Final Fantasy 7 remake.
He explains, “This issue is related to whether a Final Fantasy 7 remake will happen or not. It is very hard to make games on PlayStation 3 in the same style as the games in that era had. Making graphics will take enormous time.”