I agree with you contra GP that "barely speak English" wildly misrepresents LLM capabilities, but I think they don't write very well, and your characterization is also an overstatement. They often use stylistic elements that don't make sense in context: pointless metaphors, abuse of bullet points and tables when a simple paragraph would do, excessive parataxis, misplaced emphasis, dense technical prose even when unnecessary, occasionally weird word choices.

Those problems are much worse in languages other than English, by the way. I'm not a native speaker but I always interact with LLMs in English because their prose in Italian is terrible, it often reads as though the text had been written in English and then translated.

Obviously there's nothing wrong with each of those stylistic elements in abstract, exactly like there's nothing wrong with em-dashes or the word "absolutely", but I would argue that writing well means choosing styles that match the content; what LLMs appear to be doing instead is forcing the message into a default pattern, which we percieve as forced and therefore tend to dislike.

To be clear: their writing is fine. The primary goal of writing is to convey meaning, and LLMs have been perfectly adequate at that pretty much from day one. I can get over their quirks.