Publishing a physical book is expensive, publishing a website is dirt cheap. For the publisher and author of a book it’s much more imperative to release good content that people will want to read, cover to cover. You can find more extensive, in-depth, and up-to-date content on the Internet but where exactly and is the info all in one place? Websites are also often made with the assumption you’ll jump around or that you know what you’re looking for. If you’re learning something new it’s often best to have a linear, paved path and not be your own guide.

Edit: Someone else said it first but I was trying to say library books are “curated” in a roundabout way.