Professional moderation is expensive. Keeping HN as good as it is is literally dang's job, paid for by YCombinator (who benefit from it by using it to recruit and then promote startups).
This makes sense, although it clearly isn't enough to induce HN-like moderation. Surely there are professional moderators on similar platforms such as Reddit (besides just their volunteer subreddit moderators), and there are certainly professional moderators on Facebook as well given their revenue streams. Neither website has arrived at something like Hacker News, at least as I value it. Part of this might be the scale of those websites because it's presumably difficult to scale up moderation teams that require human training.