Because GPT-2 in its current form isn't that great at keeping track of facts (like the player's inventory or skill points) over the course of long conversations

Because GPT-2 in its current form isn't that great at keeping track of facts (like the player's inventory or skill points) over the course of long conversations, or at consistently enforcing rules (as the OP's blog post demonstrates). Allowing a more logically stringent system to handle those parts of the game might produce more realistic results.