Religious charity also doesn't really explain it. Even if you removed 100% of the U.S. charitable giving to religious organizations, they would still easily ran

Religious charity also doesn't really explain it. Even if you removed 100% of the U.S. charitable giving to religious organizations, they would still easily rank #1. My guess is that it's mostly cultural. For example, a lot of large employers will even match employee contributions up to a certain level every year to the charity of the employees choice.