Some of us been writing texts on the public internet for decades, and humans invented machines trained on our texts, so suddenly the text we write now sounds like robots? Only way to win is to stop caring, let the people believe you're a LLM if so be it.
Ultimately it's a mix-match of everything, including whatever data the pre-training uses and how exactly they do the post-training. I don't think you can say there is a single factor that decides the writing style, unless you have some particular insight into some specific pipeline. Generally though, they output text that looks like the human text they ingested for training.
