LLMs can't tell the difference between code and comments in some cases. Older code bases that have had a number of hands touching it over the years have a lot of commented out code and dead methods. LLM sees that as good live code. Also there will be comments like 'this is a stupid way to do this, but I don't have time to fix it right now'. LLM has no sense of humor in this case.
And sure. I should clean all this up before hand, but who has time for that? Project has one, part time developer working on it now.