Why People Knows The Right Way In Investment & He Willing To Teach Others While He Can Make A Lot Of Money?

Why some smart people knows the way about investment and he could make a lot of money, BUT still want to teach others how to invest?
For example, like some people play stock market. They willing to tell people what would be the trend of a stock or they would write a book to let other people know how to make money.
I don’t understand. If they kept the serect, they could make so much money to themselve. Why tell other how to do?