The problem is that a lot of the time the people who are scammed are people down on their luck, trying to find a job, or any money they can get. As well, the people shilling the schemes are often very charismatic, and are good at painting a picture of assured investment being paid back. But usually that is not the case unless you're the one at the top.
All it is, is getting someone below you to do the job for you, and it keeps going down this pyramid until you have hundreds or more under you. All the while, the people at top reap the rewards, while the people at the bottom have to work a ton just to make it.
People invest because these people up top try to make them think they will earn a profit from your investment fast, and that's never usually the case.
The best suggestion I can give, is this. If it's too good to be true, it's likely not real. Nothing comes as easy as you'd think, especially when it comes to making money. Research something before you put any money into something. Even if you think it's fine, research it through and through. Make sure you check everything you can before you invest any of your hard earned money.