Show me one example where government is able to stimulate economy. First, you will have to clarify what 'stimulating an economy' means. Once you have specified what it means, you will have to explain how each dollar spent has somehow stimulated the economy.
I think you fail to comprehend how complicated modern day economy is. Have you heard of 'I, Pencil' story? http://www.youtube.com/watch?v=d6vjrzUplWU
The gist is that no single person knows how to make a pencil. Because of co-ordinated efforts of thousands of people, working for their own goals, makes it possible to manufacture pencil and deliver it at your local store where you can buy it for dollar a piece.
Do you really think that few hundred bureaucrats in DC can decide how billions of dollars should be spent so that economy is stimulated?