Because this isn't a magic fantasy world where you're literally turning cash into bigger cash. When someone says "$5 cash becomes $15 worth of food", they mean "I can spend the $5 to get what would cost you $15 to spend". And when it's broken down into "of the $5, $3 goes to paying for food, and $2 goes to paying people", that $2 pays the people who can use the $3 as effectively as the original person could use $15. If you decide to get rid of those people, and just spend the $2 directly on food, you can no longer use it effectively.
