Because even if you have an edge at a game of skill and chance, it's not a license to print money. Professional blackjack players have to fly under the radar, and even with the large bankroll needed to survive swings, can only expect to make somewhere in the 5 figures. Plus, you have to spend all your time playing blackjack. Wouldn't you rather sell a book?

Playing blackjack is really, really, really boring (really!). There's a reason professional gamblers call it a 'grind'.

There are mathematical systems to beat blackjack (counting cards, change your bets when the odds are in/out of your favor). Are you doubting this?

The number of people that are going to successfully read the book, count cards, and beat the author are few to zero. The number that will buy the book? Many. Heck, I've read poker books, but don't play poker. It's interesting to learn algorithms, reasoning, and also to consider the psychological elements.

As an aside, blackjack is beatable if you count cards. The books are not bogus (not sure if you are implying that or not).

Daily Fantasy Sports is not like blackjack, because how well your fellow players do directly impacts your chances of winning.

Because it's passive income.

You play against the house in blackjack, you're not playing against other people who read your book so it doesn't hurt his chances at winning.