The main reason is that when a trade goes against you and it approaches your physical or mental stop loss, you know if you let the trade hit that stop you will lose that amount of money. So sometimes traders will either have a wider stop or even move their stop down hoping that price will turn back in their direction and that they will not be stopped out.
The reverse is true for winners. Once a trade moves in your direction you know if you kill the trade or lower your target you will 100% have a winning trade and book that profit. Plus the fear is the reverse of a trade approaching a stop, you think if I don't take the profit, price will reverse and the trade will go all the way back past my entry point and hit my stop. Yes, this does happen, but what can also be done is once the trade is going your direction instead of booking the profit early, you can move your stop loss up a little but make sure the trade still has room to breath.
The other problem is that if you have a 50% win ratio but you let your losses run more than your winners assuming standard target and stop, you will over time lose more money.
A good solution is to have a standard stop and target based on back tested price swings so that you know how far to place your target and stop. If you then are able to achieve say a 60% win ratio or higher with equal risk vs reward then over time you will make money. However, for me this is not set in stone. If I see that I need to move my stop a tiny bit more to make sure that the market does not prove my original trade wrong I will do so. If it looks like you need a big stop for a trade then you are not getting long or short at the right place. Also, if I see that my standard target is unlikely to be hit say because price will need to break the high or low of the day, I am willing to lower it a little. Also, you need to determine if you are trading at a good entry point vs chasing price. You need to have patience to wait for your setup.
