Probably the code of these games leaked in some dark corner of the web in the past and that’s why AI can generate it so nicely. AI is also extremely good at building Quake 3, including Carmack’s notes.

But as soon as you are just a tiny bit off the beaten path, Fable/Sol will go postal regularly. So if your indie game has interesting gameplay mechanics, nobody will be able to 1-shot that for a long time. (Because their prompt would have to include everything that’s unique to your game.)