It is actually. It's been shown that longer books make more sales as they are considered more trustworthy, so authors are incentivized to artificially drag them longer than they actually require
The money isn't from book sales. The money is you can charge higher consultant fees because "you wrote the book". If you don't play the game of course you won't make money, but writing a book is one step. (the full game has lots of different paths, there are other ways to make a lot of money without writing a book)