The US has almost 4,500 banks and about the same number of credit unions. They're subject to federal and state regulation. That's a lot of complexity to manage.

However, there's FedNow immediate transfers, Zelle (consortium of banks) immediate transfers, same day ach (several batches daily), same day fed wire transfers within banking hours, and that's just the faster options.

Decentralized systemd are slow to act and slow to change, which is why all of the faster banking initiatives tend to involve a centralized element.