I am never quite sure whether it's pure greed or ignorance that make people believe these things are possible.
One of my benchmarks I've always suggested people use is the "Credit Card Test." If you figure the absolutely worst credit card charges you 29.9% interest a year, you should figure there is NO investment on the planet that can "promise" you higher returns than that...
The other test-- which people generally refuse to look at-- is that if it were truly "so easy" to make $5000 a day (or whatever) do you honestly think a single person on the planet would actually go to work on a daily basis?