depends on the book? I've read lots of books where it turned out the author had effectively one idea, it should have been 1 chapter, but they turned it into a book with 24 chapters of filler.

I've thought about this a lot. But my conclusion is that many times the base idea is valuable but the author spends time using examples and different use cases to show you the effectiveness of the idea and a wider range of uses.

Of course this isn't always true but it's true quite often.

Take one random example - Spark: The Revolutionary Science of Exercise and the Brain.

The idea is in the title. You don't need to read more than that to benefit from the idea. But all the different varieties of benefit and pathways and studies the author sites are still valuable.

So you've read a lot of bad books.

Productivity hacks and pop psychology are not what we're talking about here. We're talking about interesting works of non-fiction. And if it's fiction, and you think that there is "one idea" and you can skip the rest, I don't know what to tell you.