The AI models (not specifically OpenAI's models) do not contain the original material they were trained on. Just like the creators of Undertale consumed the games they were inspired by into their brain, and learned from them, so did the AI learn from the material it was trained on and learned how to make similar yet distinctly different output. You do not need a permissive license to learn from something once it has been publicized.

You can't just put your artwork up on a wall and then demand every person who looks at it to not learn from it while simultaneously allowing them to look at it because you have a license that says learning from it is not allowed - that's insane and hence why (as far as I know) no legal system acknowledges that as a legal defense.