Because they collect their 3% or whatever, and even after giving you your "cut" for strongarming the merchant for them, they still get 1% or whatever it is.

You've paid off every month on time for 20 years, and they get 1% of that. For basically running some servers and balancing payments.

That's big money - debit cards do the same thing for what, 25 cents a payment? So once the credit card hits about $9 it's all gravy.