Accountability. Same reason I want to read human written content rather than obvious AI: both can be equally shit, but at least with humans there's a high probability of the aspirational quality of wanting to be considered "good"
With AI I have no way of telling if it was from a one line prompt or hundreds. I have to assume it was one line by default if there's no human sticking their neck out for it.
The human who submitted the PR is 100% accountable either way, thats partly my point.
Disclosing AI has its purposes, I agree, but its not like we can reliably get everyone to do it anyway, which also leads me to thinking this way.