Actually the programming isn't complex, there's nothing more complex in it than the rest of the content the game already has. But it would involve additional game design, lots of time spent on creating and modifying the buildings (the buildings aren't fully drawn, the "back sides" are blank areas), it would also make the game slightly more heavy due to the additional graphics, and finally it won't add anything to the game. It would just make it more easy, whereas currently there's a challenge of optimizing your city space with pre-defined sized. Like tetris, for example.
I could disagree with some development choices for the game, but not that one.