There is a simple reason that self-help books continue to sell:
We can't get this data through our schools or "normal channels". People aren't taught how to do this stuff - you know, make themselves a success, achieve the "American Dream" and all that.
But it is native to each of us. We all want to be better.
So when various authors come along and write very useful books on the subject, then these books sell well. People want to have the data which supports their native urges. (Ever wonder why "sex" is used to sell magazines, cars, clothes, etc.?