By your logic, few people get wealthy off their businesses, so hey, don't start a business.
The reality is you have to figure out what is going to sell. I have no interest in writing books, but if I did, and I wanted to make money. You can bet it's not going to be a technical book. I'll be going after a market that is large and hot, think books like Harry Porter or 50 shades of gray.
Make no mistake about it, the business doesn't start until the book is done, the business is not writing the book but selling the book. Most authors just write and then sit and hope it get's popular. You have to hustle and push and sell and sell, so it get's popular.
I watched MJ write this book, he mentioned he was going to write it on his forum, then he did. Did he make any money? Nope! Then he got into marketing/sales mode and pushed it, and it sold. Got lot's of review on amazon and sales took off, I don't know his numbers but I betcha it's much much larger than your average technical books.
I won't be surprised if he is well into the 5 digits. He also self published, so he's collecting pretty much most of the income. How did I reach 10k? 4 hour workweek has sold about 1,350,000 copies and has almost ~3k reviews, amazon MJ's book has a little bit 1/6th ~500+ reviews on amazon. They both have 4.5/5 stars.