I agree to your point as there are factors that really change the mentality of some traders, both greed and fear that comes out when they are in the process and while waiting for the market to move, it's no an easy task following your set limits or following the strategy that supposedly you are planning to execute. Temptation and such fear inside you can change the direction of your target and may lead you losing your position and the amount that you set for your investment.
I think the reason is that it is a very difficult job that requires a lot from the trader, perseverance, discipline, and many other rules that cannot be broken. I think that in trading you can achieve profit, but often it is a small profit, because they are not ready to trade big money, or are carried away by futures, where I think it is very difficult to achieve positive results. Another problem is that everyone wants to get a quick result, and in trading this is unlikely.