Part of the reason it works so well is that SEO people who are trying to manipulate the rankings of their site don't know the details of the algorithm. At best

Part of the reason it works so well is that SEO people who are trying to manipulate the rankings of their site don't know the details of the algorithm. At best they can guess and run experiments. Google making their algorithm public would result in it being much easier for website owners to inflate their rankings, resulting in lower quality search results, and giving companies who pay enough money to SEO experts an unfair advantage (far more so than is already the case today).