Because I'm scared of how much extra work it would create, and because it's the sort of frog-boiling bureaucratism that it is in the spirit of this site to avoid. It's easy for me to imagine that one day I would wake up and be horrified by the systems we had inadvertently let sprout up around ourselves.
HN works best with informal systems, not formal ones. The informal contract around transparency is: people are welcome to ask questions and we always answer them, but we don't formally publish a moderation log or anything like that (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...).