They really don't, if you actually bother prompting them. Give them a voice sample, and tell them to match the tone, and you already get something 10x better. Have them revise with a list of common writing problems - not just common LLM patterns, but guidelines for writing better - and you get rid of more.
Properly prompted, an LLM writes far better than most people.
Properly prompted, an LLM writes far better than most people.