Why do so many people in this country believe that the president (or the executive branch in general) has anything to do with the economy?
Personally, I beleive it is due to the general public complete lack of understanding of economic theory. A capitalist system is cyclical by nature and any interference is nearly certain to be miscalculated and lagging at best.
For example, the "economic stimulus package" will have absolutely no effect on the economy in the long run, other than increasing taxes for future generations. It is just a political tool to take advantage of the ignorance of the people.