I imagine it's because of the style they're going for. Painted details without much dynamic lighting. Also, these days, shaders have as much if not more of an impact on performance than polygons do, so I imagine it's not as much of a performance hit as you'd expect.
Ahh... I thought this would be bigger model hence why I thought normal mapping.
I work (Used to I am on sick leave now) as environment designer and artist. I am used to build thing in such massive scale that most of it don't need anything else than normal maps because player never gets close enough the material I ave done.
My complex architecture used to piss off our texture artist quite a bit and the art designer used to piss me of with her ideas.
Might as well mention also, we can probably assume this is the high res version. There are a lot of places where polygons could be optimized, but such operations will be inherent in the triangle reduction performed either by the game engine or manually when creating the LODs for the game.