Returns elsewhere are not that good. If interest rates are negative you might as well put it in a mattress (hoard). It's only $1.7 trillion... meh.
Of more concern is $1.2 quadrillion in derivatives. No one really understands what would happen or where the bottom is if something causes that market to unwind.
"Wilmott believes that derivatives represent a risk of unknown proportions."http://www.aol.com/article/2010/06/09/risk-quadrillion-derivatives-market-gdp/19509184/