Because Morno throwing money is instant 'stimulus' whereas infrastructure takes a long time to put in place - especially in this modern era of infrastructure projects being endlessly delayed and the costs driven up by Environmental Impact Studies/Statements.
I think you will find that the SHB was NOT a depression era stimulus project - a piece of infrastructure like that would have been in the planning stages for a decade or more before the Great Depression. You will also find that the Australian Govt of the day did NOT take the Keynesian approach during the GD but actually the opposite (dramatically cut spending/public service wages etc) and as a result Australia was exiting from the GD after only a couple of years - a full decade + ahead of the Yanks - who went Keynesian.
Its interesting to compare recessions/corrections that occurred before Govts got hold of a large chunk of the economy (through personal taxation - around 1913 in the US) to those after. Beforehand Govts could do nothing and they were short lived events that relieved the economy of its imbalances and away it went again - after they started to be able to interfere they of course did and the results have been, without exception, truly bad. The current GFC is a product of political interference in the previous several economic corrections making them smaller than they otherwise would have been - the Tech Stock crash is an example. Greenspan lowered interest rates to silly levels and left them there thus turning a correction that would have effected very few people into a housing bubble that effected everyone.
I would have to agree that Howard's workplace reforms cost him Govt - Labor's FWA is no better + we have all the other baggage the economy is currently being saddled with. Personally I believe Australia would have been in a MUCH better place right now had Howard/Costello won. What is economically more sustainable - a short blip of higher unemployment (debatable if it would even have happened) in a structurally sound economy or the massive stimulus debt/wastage and Australia being made a laughing stock with the proposed new super profits tax?