Because who wants to take on the risk of building an asset, using inflated building costs to the large crack like sound of a large bubble bursting.
If the government had policy setting right, this figure would never get this volatile. Like if they set policy to actually encourage the building of new homes rather than buying votes from property holders and creating asset bubbles.
The possible result, in fact the deserved result is the entire economy directly and indirectly connected with building houses will be damaged so badly that the resulting unemployment may greatly magnify any associated recession. Stupid policy deserves disasterous outcomes. Politicians try and buy votes, it works short term but the ferryman must be paid eventually.
