Often? What conceivable (legal) system could consistently generate such returns and not manage to capture all capital on earth eventually? The best financial vehicle ever conceived has about 9% per year over the last 50 years or so.
If you know what you are doing and have luck you can make very high returns.
Mostly people don't know what they are doing though and get into big trouble.
Well, obviously. You can gamble on red in the casino and get 100% return (with a 50%+ risk of losing everything).
The point is that these scammers presented the 18% APY gambles as virtually risk free "investments".