In comparing different types of investments, stocks stand alone in their ability to build wealth. Companies can reinvest their earnings with the expectation that they will be able to create even more future earnings. This is the reason the S&P 500 has been able to push new highs since mid-2012*. Not because there are now 600 companies in the S&P 500, but because the existing companies successfully reinvested in themselves and increased current and future expected earnings. Eddy Elfenbein contrasts this with other investment types in Equity Versus Assets.
*S&P 500 Total Return Index