Because the economy is still in the toilet and most people either don't have any money to invest or are smart enough to stay out. There are nothing but institutional investors playing the market now. This 13K DJI number is absolutely nothing to be impressed about.
But 13k is an important Psychological watermark! /S
That, and the fact that the Fed and the other centrtal banks have pumped about $7 trillion in liquidity into the markets over the past couple of years...