It's used a lot in formal writing (academic papers, books etc) which are probably a large portion of chatGPTs training. If the HRL was done by professional writers then it was probably additionally biased toward using them.

People are more casual on the web. It's sort of like how people can often tell when it's me in IM without my name because I properly use periods while that's unusual in that medium. ChatGPT is so correct it feels robotic.

It’s the most likely explanation I believe. I have no idea about the content distribution of the training data but I would have assumed twitter and Reddit content would completely dwarf the literary content. Somewhat good that if it’s indeed not the case!