Reason being, a high proportion of AAA game development is done using off-the-shelf game engines. Of course it's still necessary to write code for the game logic and features, but because of the stable base this code is written against, it might not be seen as necessary to follow a strict TDD/BDD approach.
I don't think this is particularly true. A high proportion of game development is, but many AAA studios are using either home-rolled engines, or started with an existing engine (Unreal or iDTech) and what they're currently running looks absolutely nothing like the original engine.
The problem is, most of the major game dev companies have their own engines. Unreal Engine is massive in AAA games, but most EA titles use Frostbite, Valve: Source, Ubisoft: Anvil, Crytek: CryEngine, Bethesda: Gamebryo, and the CoD devs use IW, which is a fork of id tech engine.
However, this doesn't mean that they aren't using off-the-shelf engines. As long as you count in-house "shelves".