> Payments are transferring bits, and should be effectively free.
Payments is taking on a short term risk that the (slow) transaction between a customer's bank and a vendor's bank will not settle, because of (a) fraud, (b) chargeback-like issues, (c) insolvency. This isn't exactly right, but overally its not quite 0 risk without crypto, which people don't trust (from a UX perspective).
And all kinds of risk-taking charges for it: loans, mortgages, etc. If you want to shoulder the risk yourself and save the fees, pay with a debit card.