I think the idea of trickle down economics springs from the idea that rich people, having made a lot of money from their investments or high salaries/bonuses tend to spend the money, buying things such as houses, cars, jewelry, etc., which means that they are injecting money into the system, fueling the churning of money, as it turns over several times. Conversely, a very poor person living in subsidized housing and getting food stamps does not buy an expensive house, a new car, take trips to Europe, etc. He/she needs every cent just to sustain his/her existence.