One reason that I think is reasonable enough to support your statement is because traders often forget all their planning when they see a profit opportunity when they are already in the market, some natural traits often interfere and affect all the planning that they have previously implemented such as greed, inability to accept the risk of loss or doubt when opening a trade which in the end usually all of that leads them to losses.
But I think that is something natural because in any case a beginner must need time to adapt to every positive or negative situation they experience, experience will tell them and something that was initially unusual will eventually become accustomed to over time if they can be consistent.
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.
For the problem of greed, I think one thing that traders must correct is to re-understand what and how trading activities actually are so that one day they will no longer be trapped in greed, one of which is understanding that trading is not always profitable because the market cannot always be predicted 100% accurately and for the problem of often violating planning, I think it is clear that the key is to be more disciplined in your planning.
The point in trading, in my opinion, is to never learn new things and also continue to improve risk management and evaluate mistakes that make you lose.