$1 per day isn't much money as far as initial investments go. They are also using the S&P 500 which by it's nature is mostly mature companies that aren't typically going to see explosive price doubling growth.
The S&P 500 is also a much safer hands-off investment which is likely to beat most active investors - especially for the amounts of investment we're talking about for $1/day (or even $10-100/day...)