I don’t quite understand why they don’t just let trusted users manually rate search results and feed that through AI to determine ranking penalties similar but not exactly like Kagi.
They're already doing that with paid quality raters. I suppose your question is about opening this up more widely, but that's basically giving those spammers a tool to directly influence the ranking, which is going to be even worse.
- Sergey Brin and Lawrence Page, The Anatomy of a Large-Scale Hypertextual Web Search Engine