They're are plenty of mods that inject AI into games, like Skyrim. Though in order for a game to have LLMs and scale to a shipping audience without a subscription, you would want to run them locally.
The real technical blocker is performance and voice synthesis. If voice synthesis was at parity with human actors, it would be worth it to battle the performance aspect for major studios. In text based games especially, taking time to generate enough text is just too slow to be convenient
Yeh there is a skyrim mod that lets you talk to any NPC and basically queries an LLM behind the scenes even with a screenshot of the scene so it can react to your clothing etc. if you insult it the LLM understands it dynamically and makes the NPC attack.
I mean thats not a new game concept but I definitely think that levels up the experience.