In Square Enix's book "Final Fantasy XIII Scenario Ultimania", it is revealed that the reason why there are essentially no towns in Final Fantasy XIII is because it is very difficult to make towns on an HD console. Here is the direct quote:
The game’s linearity was just because depicting towns and so on like we did before was impossible to do on an “HD” console – it was too much work.
http://www.ff-xiii.net/index.php?page=article&article=No-Towns-Because-They-Are-Too-Much-Work--514