The rule is necessary because the maintainers want to build good will with contributors and if a contributor makes a bad PR but could still learn from it then they will put effort into it. It's a "if you made a good effort, we'll give you a good effort" and using AI tools gives you a very low floor for what "effort" is.
If you make a PR where you just used AI, it seems to work, but didn't go further then the maintainers can go "well I had a look, it looks bad, you didn't put effort in, I'm not going to coach you through this". But if you make a PR where you go "I used AI to learn about X then tried to implement X myself with AI writing some of it" then the maintainers can go "well this PR doesn't look good quality but looks like you tried, we can give some good feedback but still reject it".
In a world without AI, if they were getting a lot of PRs from people who obviously didn't spend any time on their PRs then maybe they would have a "tell us how long this change took you" disclosure as well.
If you make a PR where you just used AI, it seems to work, but didn't go further then the maintainers can go "well I had a look, it looks bad, you didn't put effort in, I'm not going to coach you through this". But if you make a PR where you go "I used AI to learn about X then tried to implement X myself with AI writing some of it" then the maintainers can go "well this PR doesn't look good quality but looks like you tried, we can give some good feedback but still reject it".
In a world without AI, if they were getting a lot of PRs from people who obviously didn't spend any time on their PRs then maybe they would have a "tell us how long this change took you" disclosure as well.