The reason people think such an outcome is unlikely is that the loss of wealth during 2008 and the need for private-sector balance sheets to be deleveraged are thought likely to constrain both household demand and the ability of the banking sector to expand credit, for some time.
This is a reasonable reading of the history of most financial crises, and underpins the consensus forecast that global growth, when it resumes, will be pretty modest initially. Yet the speed and size of the responses to the downturn by policy-makers around the world is just as unprecedented as the speed and size of the downturn itself.