They are not just pledging money away. These are serious investments that are evaluated in the same way a VC/Angel pick a startup. A colleague of mine tried to apply/pitch to the Bill and Melinda Gates Foundation and the event (many people pitched) lasted six hours with two reviews and rigorous questioning. He was not successful, but he said it was a very thorough examination.^1
Bottom line: A portion of these pledges are more than putting money into collection baskets, it is putting it into the baskets of third world women who need an extra sewing machine to make ten more shirts so their kids can go to school and have food.
1 - I am sure not every donation is made this way