There are many financial gurus selling their books within thir courses on the internet. The purpose is always the same: financial freedom for the average person.
After all we reach to the conclusion these books are a copy of the previous titles. Authors select catchphrases they read previously in books from other authors which create a powerful impact on readers' minds, added to personal testimonies of pain, resilience and success.
And then, as OP claims, they become an authority on this niche, at least for the public who falls for these cheap tricks...
On the other hand, I must be fair and state there could be some precious lessons and teachings on some of these books.
I love those books though, but then again, even if you read all of them till the last page, but if you failed to execute all the learning, then it's nothing. So to become the authority in any field, you really had to get your hands dirty, your feet wet to get and feel on what it is, and what you really want to accomplished.
And this author become the authority but just doing that, they try to learn it by themselves and then sharing it to others, whether to gain financial freedom and others things in life. But the most important thing is that we should enjoy what we are doing, maybe we are motivated, but at the end of the day, life is hard but at least when we traverse that journey, we should learn how to smile and enjoy.