Thats because most can't time the market. Its very easy to tell when to put money in. The current economic climate is a perfect example of a time to buy. Its very hard to tell when to get out though. Human nature makes you want to keep going for the home run, but those are very rare. People make careers out of manipulating the markets and making amatures sell when they should buy and buy when they should sell. They are called "market makers" they are paid to make money for brokers, corps, and some others by creating hype, moving numbers around and messing things up in general (wouldn't be surprised if this practice gets banned in coming years.)