If I make $100 selling stuff, you give me $1 to donate, and I donate that dollar. I’d tell the IRS I made $101 and donated $1 to charity. Getting that $1 written off. Effectively telling them I made $100 in profit. If I didn’t get any donations I’d say, hey I made $100 in profit. Where is the part where they profit off the dollar? They can’t donate to themselves.
This is why you can also tell the IRS that you donated money, because you did.