Most traders dont fail at trading because it is hard. They fail because they are not willing to put in the work to create a detailed plan, test the plan, and then follow the plan.
I don't agree that traders fail because they don't put work or create detailed plan, they do but because it doesn't work out doesn't mean they don't but what I believe is that it is a process of learning and mastering of trade takes time. Trading is not about detailed plan but you must have the confidence and emotion required for trading and if you don't have control over your emotions then you will struggle for more longer time.
Different traders will fail for different reasons, a newbie which has not read a single book about trading will lose their money because of their ignorance about the risks they are taking, while a trader which has read several books and created a good strategy most likely will lose because of their inability to follow their own strategy no matter what.
As such depending on the reason of their failure the actions needed to correct those outcomes will be different, however taking into account what I see in the forum with those which are newbies, I really think the majority of them do in fact lose their money due to a lack of understanding about the markets, as they believe they can make a lot of money without the need to invest their time and effort to understand what trading is really about.