Because it's gambling. Some are better gamblers than others, but no individual can consistently have more ups than downs over a period of years. "I was making

Because it's gambling. Some are better gamblers than others, but no individual can consistently have more ups than downs over a period of years. "I was making a lot of money but now I've stopped" is the same thing as "I was lucky until I wasn't". Making a living by gambling pretty much sucks, which is why most hackers don't do it. (You'd think that something as complex as markets would attract hackers trying to "figure it out". The problem is that due to the changing nature of the other participants, all hacks are temporary. Makes for great blog posts, but not a long term strategy.)