Likely because, in 99% of cases, it was. I doubt anyone would willingly leave the co-author advertisement (because that's what it is, an advertisement) on display in all their commits unless they've gone all-in on the fad and are actively proud of the fact that they're not writing any code themselves.
That said, I don't think this is a bad thing. It helps signal which projects should be avoided if you care about quality at all.
Agree, though once a commit is pushed it's too late to remove it without rewriting history, which is a sin much worse than forgetting to remove it. I frequently use Claude to commit work that I have written, because LLMs are really, really good at writing commit messages. My muscle memory early on sometimes ran gp (my alias for git push) instead of gca (my alias for git commit --amend) and unintentionally pushed. Even though I had written the changes myself (not used Claude for the code), it made it look like I vibed it (which really pissed me off btw. I'm still mad about it. I despise some company injecting ads into my work)