Most people lose money on trading systems, this is because they quit during a string of losses. For discretionary traders, there are also no magical indicators, which generate good signals all the time. The trader must learn to know when an indicator should be followed, and when it should be ignored. Some people are far better suited to mechanical systems trading than discretionary trading. It requires complete confidence in the system, and suppression of emotions, the trader must execute every signal.
Discretionary trading can be very rewarding.
But it is hard work, and can also be very stressful. Even when you reach a degree of comfort with your style, the learning process never ends. Trading is not a low-stress easy occupation. In order to trade with consistency, they design a system that actually reduces the amount of discretion in their trading, but they retain input in key areas of their plan. Backtesting and paper trading are valuable tools.