Why do most self-help books seem like they could have been blog posts? This isn't true for every non-fiction self-help book, but it's definitely the case with plenty of new releases.
Most books are just repetition of their table of contents. You can read the index of a book and probably get away without reading the whole thing.
A few years ago, when I was thinking of writing a non-fiction self-help book, I started with an idea, turned that idea into a long blog post, and then intentionally broke it down into chapters. So that's how these types of books are written.
Yes, most of these books could be condensed into a blog post and convey all the points the author wants to make. That's why I prefer reading summaries of non-fiction self-help books these days.
I know you could argue that the repetition and redundancy is meant to drive home the message, but I'd argue back that treating a book summary as your notes and reading those notes multiple times would be more fruitful.