“Yes, banks have the functionality and capability, but launching something new is often a case of speed and cost,” said Danilkis. “The banks might have a generation-2 system but many will be much older. And  changing how a financial product behaves is very difficult and highly risky because even a small change can create problems. And those systems are not designed to work with APIs, so it is extremely hard if not impossible to connect to other systems, never mind in real time. Certain solutions or offerings become impossible or impractical to build yourself.”

https://techcrunch.com/2021/01/07/mambu-raises-135m-at-a-2b-valuation-for-an-saas-platform-that-powers-banking-services/