That is not true. The portfolio that I have shared here is giving around 18% CAGR. It is possible to obtain over 20% through long term investing. It is also possible to make money day trading, manually as well as algorithmically. You can read about those who have been doing this successfully in market wizards book series.
I believe that the difference lies in risk first approach. Most people trade focussed on gain. They become impatient and want to earn big money quick, and in doing so lose sight of the losses they will take if they are wrong. If they are lucky, they manage to avoid big loss for some time, but luck runs out eventually and they end up losing an amount they can’t afford to lose.