When people say something like that they actually mean going through the trouble of learning why experts do what they do and then try to make their knowledge and their skills your own, copy trading is just a cheap imitation of this process and it will never work as intended.
As do you think that a world class trader will allow themselves to be copied for a single second? If you want to have the privilege of your money following their strategy then you have no other option but to hire their services, which are not cheap, and you need to have a massive amount of money just to have a chance to hire them, as there are countless people which would like for the privilege of their money being managed by them.
Moreover, if we are talking about a successful trader who can earn a lot of money, then what is the point of him sharing his knowledge with anyone else? My opinion is that trading is taught by those who have not been able to achieve great results in trading, but at the same time they believe that they have quite a lot of knowledge in technical analysis.
Thus, for them, it becomes the realization of their knowledge, they simply teach beginners what they know themselves, but the result in trading will still depend on the beginner, because in the end he will have to analyze and make decisions himself.
Copy trading is also not something to strive for, what will it give a beginner in the future? No personal practice and understanding of the market, at best, a few successful copied trades and that's it. And when you have to trade yourself, then everything will be different, you need to make decisions, this is a completely different matter.