In that case, only getting a few, hand written PRs could mean getting fewer, but higher quality contributers.
Other projects might not care about getting many contributers, many stars or being widely used. It might simply be a small project someone wants to code by hand. That's absolutely valid, many people still have fun coding by hand.
The truth is just that there are a lot of slop PRs currently, where ppl just sicced an LLM on a problem, did not validate any findings, did not check the code, did not read the output and just acted as a meat proxy. In many cases, these bans on LLM PRs are simply a reaction to this flood of PRs. After all, as I said above - everyone can do those prompts themselves, they don't need a PR for that.
Sure, it might hurt some vibecoder feelings that "Claude, write a useful contribution to this project, make no mistakes" returns in them getting banned rather than a mention as a valuable new contributor of a load-bearing OSS component, but that's a price many projects are happy to pay. Any genuinely valuable new contributor would have no trouble spending five minutes writing a PR in their own voice, before asking volunteers to spend fifty minutes reviewing it.
That is the purpose.
If I am maintaining a project, I don't need your LLM contributions. I can prompt on my own.
Now, if you have some insight to offer that a LLM cannot give me, then I might listen.