It's the same as the reason why people like Pat Flynn make 25x more from affiliate fees than from any of their actual businesses. There are a lot more people who want to "make money" than there are people who want to "start and grow a business."

PF makes money selling tools that help people make money online (doing anything - read some of his article on "article spinning" and how he defends it and it will make you want to vomit). Darren Rowse makes money selling tools that help people make money blogging. Even Nathan Barry (who I'm a big fan of) made a lot more money from Authority than he ever did from his design books. ConvertKit was just a shift from product to service.

Along the same vein, a lot of people in Amway want to "make money" and not very many want to "start and grow an MLM business." Selling $40 CDs and lecturing your down-line about buying non-Alticor products is just the best way to do it.