Same reason that stocks were reaching all time highs in 2007 when the economy was in recession. Stock prices do not do a good job of reflecting how the economy is doing. Eventually the market is always right, but until then it is always wrong.
Lots of individual investors sold at lows due to panic. Great opportunity for institutions to buy. They're the ones doing quite well. Remember, the market buys in anticipation of good news--not what is going on now. It's a leading indicator.