Being a winning poker player is very, very hard. That's why there are so few of them.
Knowing the concepts is only half the battle, and for most players it is the easier half. The second, much more difficult half is the apply those concepts with consistency, day-in and day-out. This is where most people fail.
One tilted hand, one session where you just don't feel like playing as close attention to opponents' tendencies, one time where you tell yourself "I feel like gambling, I'm due," all of these can wipe out the narrow profit margins a poker player already has.
Don't be fooled by the woofing that goes on around places like this, with the "I place 90% of my SNGs" or "I'm crushing this limit, it's easy."
The reality is, edges in poker are small. First, you are just hoping to get your money in as a favorite, and usually you aren't going to be an overwhelming favorite. Then, the small, consistent edge you do have is being killed by rake. Between luck and the rake, there's very little margin for error to be a consistent winner.