Because if you dig into biographies of these self-promotional gurus, you'll see that they only succeeded in writing books about success. They are just frauds.
Still I fill very allergic to all this positive thinking, self-help, goal-achieving and motivation literature. The skeptic in me just cannot stand it, everybody who writes it automatically gets "scam" mark from me. I'd rather suggest reading something like "Bright-sided: How the Relentless Promotion of Positive Thinking Has Undermined America" by Barbara Ehrenreich instead.