The biggest challenge for an economic thinker is to see the invisible.
Every dollar spent on one item is a dollar not spent on something else. The thousand dollars spent on a new digital camera is a thousand dollars not spent on saving, charity or a new car.
When the government spends millions on a museum or a health insurance program those millions are not spent on new housing, new businesses, capital investment and other such items. We can see the results of what was spent but we do not see the impact on what is was not spent on. We do not see it but we know it is there.
This is why the government can not create a single job, no matter how much they claim they do and will. The money for that job is money not spent by another sector of the economy that would have created jobs there.
Since the government spends that money as dictated by political power it is far less efficiently allocated that it would be if it was spent as the market dictated. Furthermore the market will be brutal in recognizing failures and reallocating it to more productive enterprises. Political bureaucracies will continue to waste money long after its failure is obvious.