That's true of bad books. But bad blog posts are even worse: they are trying to get maximum SEO. Both good posts and books are useful. When I'm getting introduced to a subject, I like books. The slower pace allows me to gradually become more familiar with the ideas and concepts. Also, good authors will put a lot of effort in writing something that's pleasant to read and that still goes through editors so that the end result is pure joy. Blog posts can be good, but I don't think they ever get to the same level of quality of a good book.

However, blog posts often has more up to date information and they are, by necessity, more direct. They are good when you are closer to a master in the subject, I feel like. They are akin to research papers, but for engineers.