Why doesn't everyone have a PHD in math? Why doesn't everyone get a perfect score on the SAT? Why can't everyone bench 500 lbs? Why can't everyone run a mile in under 4 minutes? Why doesn't everyone own a business? Why are some people entrepreneurs and some people are workers? NOT EVERYONE can be successful at trading. It takes a certain level of intelligence, a whole lot of determination and the ability to or learn how to control emotions. Very few people have the minimum required intelligence, drive/determination and the ability to control their emotions. In my opinion it requires all 3 above. If one is missing or lacking, failure is the most likely outcome. For some reason most people believe that if I can't do it nobody can or if someone else can do it I can. For example, TA doesn't work for me therefore it doesn't work for anyone. Trading fundamentals doesn't work for me therefore it doesn't work for anyone. Oh and BTW, I really doubt the number is as high as 10%. It is probably more like 3% or maybe even 1%. Trading probably has one of the highest failure rates of any business.
There's no proof or relevant study to ever prove this saying. The only study you will be able to find on the success rate of retail traders is almost 20 years old, and uses account data from a Korean Futures Exchange.
Because markets have been pretty definitively proven to be weak-form efficient and in most cases strong-form efficient, so anyone "trading" by their intuition, a data fitted backtest "system" or looking for pictures of unicorns on charts has a 10% (or whatever the actual success number turns out to be) chance of success by random probability. Which is essentially agreeing with Robert. There are inefficiencies to be found, but you need a level of rigor and expertise that the vast majority of "traders" lack, hence they succeed at the rate chance would dictate.
I think it's all about money management. Assuming your well versed on whatever your trading, if you don't know how to handle money, you're in big trouble. Think about all the companies that go under, it's not usually their widget, its poor money management. Too many traders think they're in the World Series of Poker and start doing this "all in" nonsense.
"In the short term the market is a voting machine; in the long term it's a weighing machine" - Warren Buffett That pretty much sums it up.
Amen to trade management. It's also your personality type and the type of person you are. Most people are rational. I'd say maybe 75 percent. The rest intuitive, sensor, emotional. IMO its very hard for rational people to trade. They always want to know --why? The markets behave in an enigmatic way. They are made up of irrational people who think they behave rationally. People trade their beliefs about the market not the market. All this is hard, if true, for the average person to handle. They just don't know what they don't know, especially about what's on the inside. I'm always struck by what people say when I tell them I trade. Usually I come away wondering how they survive. Pardon my arrogance I'm not that great either. But the world just doesn't work the way they were taught in school. Linda Raschke once commented about why we have such a hard time trading. We all think alike. We went.to.the same schools and learned the same things. We are not encouraged to take risks. We are encouraged to go along. We are not really supposed to question authority especially the prevailing view. Anyway there's food to be put on the table, right? The few successful traders I've known were all a little odd. But they all had a certain discipline. I think they gave up trying to put a square peg in a round hole awhile ago.