The Founding Fathers were fully aware of what they were doing when they placed the power of the purse almost exclusively with Congress. They wanted to make absolutely sure that the power of the President (any President) could never get so strong as to dominate our system of government. They had had enough of kings.
They never wanted to risk the possibility that our government could, under any circumstances, develop into an imperial form of government. Therefore, they separated the President's power of the sword from Congress' power of the purse. The power of the purse was placed firmly with Congress because our founders wanted it to serve as the ultimate check on the President.