As long as the Fed kept pumping equity into the system and managed to hold interest rates at near zero, the asset bubble has continued to inflate.
In spite of the long-term risk, there has been no other place to invest - other than stocks - that promises any reasonable rate of return.
And whenever one risk factor or another has arisen - political, currency, valuation, credit - the Fed or another Central Bank (ECB, BOJ, etc.) has acted to tamp it down by opening the money sluices and feeding the beast. In the case of our Federal Reserve, they distribute cash to their member banks who then invest it directly into the stock market through their proprietary trading desks.
Many of us in the financial services world have been warning about this for years - but no one really wants to stop the injections of monetary bliss that produce a high every time people look at their 401(k) statements.
But the withdrawal process, as anyone who has experienced it knows all too well, is going to be a bitch.