Every decent trader out there will suggest never to risk more than that. Only scammers or people who market their products and courses promise huge returns because they want to attract attention.

The reason why this percentage is so small is simple:

Even if your strategy gives you a 70% winning chance on every trade (which is a pretty good strategy BTW), the probability of having 5 or 6 losing streaks is still high. It won't happen that you have 7 wins out of every 10 trades you take. It may happen, for example, that you have 2 wins and then 6 loses and then more wins. Now if you use 10 percent of your account for each trade, those 6 consecutive loses will decimate your account.

So my guess is that those individuals who promise unrealistic returns, either don't know what they are talking, or want to attract attention for some material gain.

It may also happen that a newbie has a beginners luck, a streak of good trades, which can be even worse in some respect than a streak of bad luck. Because they won't really understand what has hit them when the luck has gone.