Keep in mind that donations to religious institutes are counted as charitable donations, whereas they can - in the best case - be more accurately described as variable membership fees. That's 39% of all donations according to the link.

Some of the donations to education institutes (19%) are also not really donations but rather bribes, aimed at receiving preferential treatment for children in admissions.

Even taking that into account, charitable giving (by non-billionaires) tends to be quite high in the US compared to other western countries. Perhaps general distrust of the government plays a role. Speaking only for myself, I could give much more to charity, but only give to the Wikimedia Foundation since by my reckoning most of the causes that should get more money (e.g., scientific research) can be handled overwhelmingly more efficiently by the government. So I would end up spending money which goes to lobbying against what I consider the better solution. I would much rather pay more taxes and vote accordingly.