I think there are a few different sub-questions here, with different answers.
If your question is, “Why doesn’t the Gates Foundation fill the entire funding gap for GiveWell’s top charities”, I think there are a number of reasons why a value-aligned, careful, thoughtful, rational, and well-funded donor might nonetheless choose not to fill the entire gap. Open Philanthropy has written quite a lot about their thoughts here, and most of that would apply to any other funder. @DavidNash’s answer seems directed at this sort of question.
If your question is, “Why does Larry Ellison give to the IDF instead of AMF”, or “Why does Carl Icahn give to NYC stuff instead of the GFI”… I think the answer is just that billionaires are just regular people, like you and me, in a lot of ways, and in particular that the things that make one a billionaire philanthropist do not really guarantee that one will be an effective philanthropist. Billionaires have the resources to get all the advice they want, but most philanthropy advice is still basically “follow your passion”. Most donors don’t even say they care about effectiveness, and I doubt the picture is much different with billionaires.
I think there are a few different sub-questions here, with different answers.
If your question is, “Why doesn’t the Gates Foundation fill the entire funding gap for GiveWell’s top charities”, I think there are a number of reasons why a value-aligned, careful, thoughtful, rational, and well-funded donor might nonetheless choose not to fill the entire gap. Open Philanthropy has written quite a lot about their thoughts here, and most of that would apply to any other funder. @DavidNash’s answer seems directed at this sort of question.
If your question is, “Why does Larry Ellison give to the IDF instead of AMF”, or “Why does Carl Icahn give to NYC stuff instead of the GFI”… I think the answer is just that billionaires are just regular people, like you and me, in a lot of ways, and in particular that the things that make one a billionaire philanthropist do not really guarantee that one will be an effective philanthropist. Billionaires have the resources to get all the advice they want, but most philanthropy advice is still basically “follow your passion”. Most donors don’t even say they care about effectiveness, and I doubt the picture is much different with billionaires.