As an author, you make very little: maybe 5% of the cover price. So money is not an incentive. You are either writing because a. it looks good on your resume, or b. you really care about this topic and feel like this book should already exist.
Books are sold based on number of pages, not quality of content. The publishers know that if your book has 100 more pages, they can tack on another $5 - $10 to the price. So there's heavy incentive to produce a lot of content.
Readers don't want to buy multiple books: they want the one book that will cover all of their needs. So they will buy mostly based on the table of contents.
So if you want to optimize for your own monetary gain, the best book you can write is one that is big, and has an impressive table of contents, but took very little time to write. So we end up with books that have a very poor signal to noise ratio. For example: my book has code samples in Python. A couple of readers have asked me to write an appendix that shows them how to set up Python on their computer, play with the REPL, etc. I think this is totally useless. They can google and get up-to-date information, and the appendix will be out of date soon. But it is very easy for me to write and would pages to the book.
So as an author, the best job you can do is pump out a bunch of fluff and then pay for reviews on amazon. It is really frustrating to see, but that's how the incentives are set up here.