Because the article is usually full of bloaty ads that take all day to load, tracking beacons, and multiple paragraphs of bloviating before getting to the point. This is a DOJ page which is a little bit different but it will still be 90% hot air and 1 sentence of substance. So people just go to the comment thread expecting to get the gist of the article through context, without clicking on it. HN's policy of having to use the original (often clickbait) title also reinforces click resistance. Being able to append a short summary (see r/savedyouaclick) would help.