Well, games are different than movies first of all.
Games are not linear; future events play out based on prior events.
The way games have worked for a LOONNGG time is that they have samples on disc (be they WAV, MP3, or something else), and the game engine mixes them all together and creates the surround experience, which is inherently PCM.
Let me also say this: There is no such thing as a Dolby Digital game! Maybe for cutscenes, but that's it. The reason many games use dolby digital is so that they can transfer multichannel sound via one connection (be it optical or coax). In the PC world, games have been multichannel PCM for basically forever. When the Nforce came out (based on the Xbox's chipset, btw), real-time encoding for Dolby Digital came about so that you didn't need 3 stereo analog cables for surround in games. Granted, the quality wasn't as good, but the connection was simplified.
Now, in this era of HDMI, there's no need whatsoever to encode a game into TrueHD or DTS-HD; it's just a waste of processing power. HDMI allows the "native" (albeit mixed in-engine) PCM to be directly transferred. The game systems of today use Dolby Digital (encoded) as HDMI isn't a given, and the next systems will probably retain that.
I hope I explained things clearly. Dolby Digital in games ONLY came about so that one could listen to surround sound in games (rather than a pro-logic approximation)...they're inherently PCM and always have been. The Dolby Digital is NOT native to the game, the xbox chipset encodes PCM into DD in real-time (on the PS3 one of the SPEs does it) for transfer to an AV receiver. Otherwise (before HDMI) the only option was multichannel analog, which is not viable for a console. With the advent of HDMI, this need is gone, so you will not see TrueHD or DTS-HD in games. You may see it in linear cutscenes to save space, but that's it.