Do you remember swine flu? Or the millennium bug? The dangers of salmonella in eggs or of cheese made from unpasteurised milk? These scare stories played for a time and were then forgotten: but cost large amounts of money and caused anxiety and loss to many individuals.
Some scares catch on: others do not. It is nonsense to claim that the dangers of the credit expansion of 2003-07 could not have been foreseen; those who did foresee problems could not attract public attention or political support for their views. Those warning of the danger of easy availability of nuclear technology and of poor control of the former Soviet Union's nuclear weapons have experienced something similar.
Successful promotion of a scare requires that some interest group benefits. Sometimes this is the scare-promoters themselves. Scientists have learnt that exaggerated claims are a route to a media profile and research funding. There is little downside in predicting disaster: if it does not materialise they can claim to have been instrumental in staving it off. Scares that thrive, such as the millennium bug and swine flu, have commercial interests that benefit from their propagation. Naysayers in the credit boom, by contrast, were trampled in the rush to share the riches available to those who denied or disregarded the dangers.