It was prevented up until the early 80's when we started de-regulating the financial sector. De-regulation led to the Savings & Loan crisis, the Dot-Com bubble, and the most recent financial crisis. Laissez-faire capitalism creates an environment for these extreme boom and bust cycles.
The icing on the cake was that these huge institutions with the money to lobby Washington got them to remove the restrictions that were unfavorable to them(i.e. repeal of Glass-Steagall), but left in place the ones to keep them afloat through taxpayer money if they gambled with it and lost.
This is a pretty good documentary that explains the recent crisis:
http://www.thisamericanlife.org/radi...-pool-of-money