Somebody would. Somebody would be an AI evangelist, or would become one. The FOSS ecosystem is large enough to be sure of that. We're not seeing nothing, we're just not seeing at all what the marketers and AInfluencers are prophesying. We're not even seeing what you describe. Why is that? Why is it limited to random commenters and not seen at all in the wild?
There is a Cloudflare project that published the entire AI generated history complete with prompts. And of course in many projects the majority of PRs are opened by dependabot, it's not an LLM but it's a "bot" at least.
I agree we're not seeing open source projects be entirely automated with LLMs yet. People still have to find issues, generate PRs (even if mostly automatic), open them, respond to comments, etc. It takes time and energy.