I think that there is a sort of mechanism by which, when a human speaks to you, you can "mirror" their internal mental state, and the quality of writing often corresponds to how much you get pleasure or information or whatever your goal is from that mental state. The important part is that the words themselves are just pointers to the state. So, the person speaking, if they are skillful, gives enough words, and enough variety, that you can start to produce a state yourself which resembles theirs.

This is the thing that LLM writing doesn't really do. Since there isn't a mental state, there's nothing to mirror anyway, and somehow you can detect the absence of it even though it's hard to put any of the machinery into words. Corporate speech also fails to activate this machinery in the same way, but LLMs seem to do it more egregiously, probably because the corporate speech was at least compiled by a human---even if it is not the thoughts of an individual, it is the "thoughts" of an "entity", the abstract corporation, which the writer was speaking for, and so you can still wrap your mind around the fact that it is communicating with you.