I would argue that ~80% is quite optimistic estimation. I think in long-term more than 99% of retail day traders lose money.
It's NOT because making money on financial markets is ridiculously difficult. It's because if you trade frequently, you get feedback from market pretty quickly. The feedback in terms of profit and loss of your money.
If you become day trader, I would guess that you want beat the market (otherwise you just buy and hold index ETF). It means that you as a day trader is willing to take risks to lose money in case you are wrong that you think you are able to beat markets.
What does it mean "to beat the market" in long-term? It means that you are smarter or faster or have some insider information. In other words it means that you have competitive edge.
Well, obviously, average person doesn't have competitive edge against hedge funds and other institutional investors who hire people with outstanding intellectual capabilities. Just read hedge fund interview questions to see how they filter people.
Other way to estimate your situation as a retail day trader: if hedge funds don't want to hire you, why do you think you are still capable to outsmart hedge funds while you can't answer their interview questions for trader role?
I don't say that it's inherently impossible to beat the market as a retail investor. It's fairly possible if you are REALLY smarter than others. And in this case, you most probably are capable to impress professional traders and investors while talking about markets. If so, they will bring you their money, and incorporate hedge fund, and you are not retail investor anymore. Just like Michael Burry did.
In other areas, ~99% of failed attempts is also true. For example, ~99% startups fail to make money. So it's normal state of affairs.