Because rules have to be enforced, and investing time and effort into detecting and punishing rule-breakers would be a pointless endeavor when, disclosure/AI or not, all code has to be reviewed anyway.

maybe we should ask the maintainers, since it's they're the ones where this disclaimer could be useful or not. :)

Because it's literally advertising for LLM companies. Because certain news sites conclude from it that "AI did $thing" instead of "engineer did $thing, using whatever tools they found useful". Because you can't actually enforce it, which makes the tag completely useless.

Any of these reasons is good enough on its own.