I wish I could remember the book that talked about this. But basically, the idea is that Christians would rather give their money to the church and then have that money be used for charitable purposes than to pay taxes, even if that amount would be less.

The main reasons are that Christians feel like they can “control” where the money goes better and, because their giving is optional, they feel good that their money is going toward something good.

Oh and it doesn’t hurt that whatever good they do is a shoehorn for them to proselytize.