Another (semi serious) possible explanation of this is that LLMs were trained on a huge volume of text from Reddit. I wonder if you'd see similar inhuman-looking behavior if you did an iterated prisoners dilemma with a group of Reddit power-users as the participants?
I had the same thought. Also, a significant amount of human knowledge/context/communication is done privately on the internet. Imagine if DMs across all platforms made it into training data...
