As you say, it's an earner for the celebrities in question. Otherwise, I think its a horribly exploitative genre. I refused to have a self-help section in the bookshop although it is one of the best selling genres.
The other problem I have with those books, even or especially, by specialists in their field, is that they are filled with padding. I really don't need to know the epiphany you had on Mount Kilimanjaro (they are rarely good travel writers), just tell me the science/research. It means you're committing four hours of your life to reading, not just the monetary cost of the book.
There's a very good video in the Zoe series about the content of this book, free and time-stamped. It's focusing on food and eating habits as Zoe is about nutrition but the examples are easy to apply to other circumstances. It's sixty-five minutes to watch all the way through, but even less if you miss the fluff and pick the stamps with the actual information. (Other videos are available).
I agree, too much padding in most of them, I'll give the video a go at some point!