Because you're the one who decided to take responsibility for it, and actually choose to PR it in its ultimate form.
What utility do the reviews/maintainers get from you marking whats written by you vs. chatgpt? Other than your ability to scapegoat the LLM?
The only thing that actually affects me (the hypothetical reviewer) and the project is the quality of the actual code, and, ideally, the presence of a contributer (you) who can actually answer for that code. The presence or absence of LLM generated code by your hand makes no difference to me or the project, why would it? Why would it affect my decision making whatsoever?
Its your code, end of story. Either that or the PR should just be rejected, because nobody is taking responsibility for it.