Simple answer: "spoiler culture" is a relatively recent phenomenon that people historically didn't give as much of a shit about as you do. Particularly with the kind of texts that are significant enough within the cultural/literary landscape to even warrant an introduction, the assumption is that your engagement with the book is being approached from an academic/analytical position. And you can't really do that, or set the stage for that, without talking about the book. If you're reading an introduction the assumption is that you're looking to engage with the text in a more robust way than simply trying to preserve the surprise of "what happens".