One of the reasons is that, most of the time, LLMs can't do graphics or write prose well enough either; otherwise it wouldn't reek of LLMs so much to people who put some effort to read it. Also, even poor writing or drawing can be elevated with some intentionality and personality, and using LLMs pretty much guarantees blandness.

You can use AI as part of your editing process. But it needs to be a process, back and forth with human input, and human editing at the end. What people primarily complain about it pure AI text. It has sentences a paragraph long using every punctuation mark invented by man. If you look at book you'll notice the same style of punctuation used but it'll be at 10% of the rate. You won't see n/m-dashes, commas, colons, semi-colons, and parentheses all used in a single sentence.

One of the most tedious ideas of recent years is the "great engineers are great writers" meme, unsurprisingly spread by writers.