These are multi year investment business cases and banks use them cross subsidise other functions they can't / dont charge for. The operational side of payments still incur operational, maintenance, compliance and administrative costs - this is common misunderstanding for people who dont work in fintech. Partly this is a result of payments being a social construct and the need to humans to agree whether a payment has actually occurred and whether the payment followed the correct process or not (spoiler alert sometimes payments dont complete properly or shouldn't not have occurred).
That banks seek to maximise profit comes down to the fact that the SA economy is largely capitalist free market (profit driven) and structurally dominated by entrenched interests. However SARBS (state) acquisition of the mandated payments clearing platform (with loudly the stated objective of driving down costs and increasing usage of real time payments) raises interesting competitive questions like is the state now competing against banks and other payment providers. Is this good or not? What is "right" in this case comes down to interpretations of the law and the states policies.