I think for that people they can predict because if they can't how they can earn more and lee lost?
Where are you getting the idea that traders always earn more than what he lose? Seriously trader is a two way activity , note this quote below ( it seems I have written this up many times )
each time someone get a profit from trading there is someone that lose something in it
Therefore you cant generalized that all traders are earning more because it is a two way activity. It is not about "knowing" the future anyway but it is rather about knowing how the market react for the current week, judging the orderbook of buy and sell order and taking some discipline to either take profit or stop loss rather than getting delusional that the price might be higher than the current one ( common mistake, holding too much when they are already on profit , should have just let it go for some small portion )
I know that, if someone earning their should losing his/her money. But also remember not always because if you or all people work hard and know the way or strategies so that they can earn and not to loss their money. Maybe your quote will be nonsense. Its all depend on person what happened to his/her money.