They charge you a large fee for every chargeback. They get the full purchase price of the transaction back without any effort. It's automatic. Meanwhile your merchant fee per transaction is directly tied to how many chargebacks you produce.

Chargebacks do not cost the payment network any money at all. All cost is borne by the merchants. That's the whole point. That's why chargebacks are effective: Because the payment network is an all powerful authority in the matter and has no incentive to deny chargebacks.