Because they are a cartel. It the marginal price for a transaction is pretty close to zero, so the fact that the price is noticable means that credit card transaction are not operating in an efficient market.
Just because the marginal price is near zero doesn't mean you're not going to pay a lot. The marginal price for software is near zero as well. You may be right about it being a cartel, but the marginal price doesn't tell you much.
