That's more because you're coming into an established site with an established topic that's been discussed for years. It doesn't really make sense to introduce

That's more because you're coming into an established site with an established topic that's been discussed for years. It doesn't really make sense to introduce every topic from scratch every time, because the assumption after a while is that the people reading have at least some prior knowledge or at least an interest in looking up what they don't know because they're there for a reason. In other words, not every site is designed around link aggregators like HN, and some things will be assumed, just as if you open a book to some random chapter.