LLM certainly does write perfectly grammatical and idiomatic English (I haven't tried enough other languages to know if this is true for, say, Japanese, too). But regular people all have their own idiosyncratic styles - words and turns of phrases they like using more than others, preferred sentence structures and lengths, different levels of politeness, deference and assertiveness, etc.

LLM output to me usually sounds very sanitised style-wise (not just content-wise), some sort of lowest-common-denominator language, which is probably why it sounds so corporate-y. I guess you can influence the style by clever prompt engineering, but I doubt you'd get a very unique style this way.