Yeah, because the human developers know the rules of chess, so it's easy to generate or verify perfect quality games at massive scale. Natural language can't be tackled like that; certainly not yet, probably not ever. Many have tried and failed to parse natural language algorithmically, but at the end of the day it seems to rely heavily on loose conventions and endless shared experiences. So, you need content from the wild, or you're basically letting the AI mark its own homework.