Because AI gets things wrong, often, in ways that can be very difficult to catch. By their very nature LLMs write text that sounds plausible enough to bypass manual review (see https://daniel.haxx.se/blog/2025/07/14/death-by-a-thousand-s...), so some find it best to avoid using it at all when writing documentation.
If it was so obviously written by AI then finding those mistakes should be easy?
Passing even correct information through an LLM may or may not taint it; it may create sentences which on first glance are similar, but may have different, imprecise meaning - specific wording may be crucial in some cases. So if the style is under question, the content is as well. And if you can write the technically correct text at first, why would you put it through another step?
Quality prose usually only becomes that after many reviews.
But why would a serious person claim that they wrote this without AI when it's obvious they used it?!
Using any tool is fine, but someone bragging about not having used a tool they actually used should make you suspicious about the amount of care that went to their work.
But if you carefully review and iterate the contributions of your writers - human or otherwise - you get a quality outcome.
But why would you trust the author to have done that when they are lying in a very obvious way about not using AI?
Using AI is fine, it's a tool, it's not bad per se. But claiming very loud you didn't use that tool when it's obvious you did is very off-putting.