Properly prompted, an LLM writes far better than most people.
Writing is a difficult skill that many (most?) educational systems do not effectively teach. Most people are terrible writers.
Maybe we shouldn't use it to write novels if we can't push it well beyond average, but you don't need to get it to produce anything more than pretty much average or a little bit better for it to be good enough in competition with average humans.
It takes some prompting to nudge the model out of that default voice because post training reinforced it. They will likely shift it once these AI-isms are known and recognized widely. I'd assume the nextgem models under training now will get negative feedback from the human evaluators for talking too AI-like and then there will be new AI smells to calibrate to.
People prefer the slop, at least until they collectively notice the AI smell, at which point the post training will likely train it out of models and slop will have new characteristics that take a while for the mainstream to detect.
There's no reason to expect a general purpose model to know what you want when you've not given it any training in what to do for your specific case.
And in this case, the models do far better than humans: Most humans can't just switch to copy arbitrary tone, just by giving them a page worth of text. We don't even need to actually train/fine-tune these models further - we just need to actually fully specify the task we give them to get them to write well.