Well, because it uses a basic instinct of man to encourage growth. Greed causes man to have ambition to excel. There are other causes of ambition such as what Socialism uses: contributing to the "greater good" of mankind (or your country), but that is subject to so many interpretations of "greater good" that make it far more dangerous. If you are not in the upper echelons of the government, your place in the "greater good" could be as a servant in their household.
JB
