Because virtually every charity I researched today, particularly the large ones that are more popular such as United Way, operate on a circular nature. They give to each other, skimming the money at each step. It is better to give your fractional amounts directly to your church or smaller charity. 

Below is a visualization of the interconnectedness - and the worst part is that it is less than 1/1000th of the interconnectedness, because I could not render the rest on one screen.