Someone's actual writing (or talking) is a rich stream of information about who they are, their motives, their preferences, modes of persuasion, and so much more. Undetectable LLM writing essentially allows someone to assume another person's identity. That is not good for anyone except the person trying to pull off a "scam" of some sort, in the broadest sense of that term. It's bad for everyone else, and for society at large.