That's because lots of banking is still fairly manual, and clearing takes 2-3 days in the United States. FedNow should help, but ACH still reigns, and transactions still take time. Ask Congress and the banks why things are still so slow.
Or, at least, that was "the new thing" when I worked as a bank teller for a summer job 15 years ago. I vaguely remember it being promised as a way to make transactions faster.