Having been in the markets for more than 10 years, I can say this statement is mostly accurate, but it requires some context.
The number we hear is that 90% of retail traders lose money. While that's not a hard and fast rule, and we might even call it an urban legend, it contains a kernel of truth that we would be unwise to ignore. Most retail traders do lose money, especially early on. Why? Because trading isn't as easy as everyone first thinks, instead it involves risk management, psychology, and the kind of discipline one might expect from a tightrope walker.
Here’s what I’ve observed:

Too many people get into trading without adequate education and preparation, treating it instead like a game of chance. They invest real money before they even know the basics.

Overextending and insufficient capitalization: Small accounts, significant leverage, and unreasonable expectations create a situation that is almost guaranteed to end in failure.

Decisions made under the influence of emotion: more accounts are lost to fear, greed, and revenge trading than to anything else, bad strategies included.

No edge or process: If you don't have a consistent edge, you're just making random bets ?and in the long run, the house (the market) always wins.

Nonetheless, there exists a small group of traders who reliably make money—that's not easy to pull off, and those who do it tend to have gone through long paths of failure, study, and trial-and-error to get there. Some do it in a way that seems to make them consistently profitable; if that's the case, it's neither an easy path nor a fast one.

It is probably accurate to say that 90% of beginners fail. But it is not accurate to suppose this is inevitable or that these were somehow destined to fail. Losing is part of the learning curve. The real winners are the ones who go through that and come out the other side.