This isn’t even the reason, because the reserve status can be immediately verified across institutions and is often backed by a sovereign in some way in case of a run and payment systems can circuit break, etc. There are legacy reasons depending on the bank network such as business hours and batching and liquidity optimization, but these are increasingly less meaningful and systems like FedNow and others offer instant and final transfer.

The real and continuing reason for the delay is to give time for repudiation and assessment of fraud, money laundering, and other financial crimes risk. The risk of instant transfer is instant theft or otherwise absconding with money that shouldn’t be yours. In fact settlement delay makes reserve problems worse because you effectively “hold” money that could potentially not be properly secured during the hold and cause a default on a transaction that was otherwise taken out of balance and pending transfer. Instant clearing and settlement makes this unambiguous. But it also makes transactions as risky as a cash transaction - instant and irrevocable.

For some customers this is legitimately ok. But by and large most customers benefit from the delays more than they’re hurt by virtue of having a window to repudiate a transaction that is illegitimate. It’s just they don’t recognize that value until they need it. We all benefit from a system that disincentivizes criminality overall. It’s hard to recognize it because we exist day to day with that benefit and it’s hard to prove the negative, but there were times without the protections against financial crimes and financial oversight and they were NOT better times. They were objectively worse, so our ancestors built a set of guard rails to prevent the endemic badness around us.

It appears though as they die off, and as we become less attuned to history, we are very busy ripping apart the guard rails our ancestors very carefully and thoughtfully built into our societies like some junior engineer who assumes every line of code written before them was written by an idiot. Take the American CDC as a case in point - the modern public health system was a very hard won victory against endemic diseases by generations - and as the generation who established it expires, we rip their legacy to tatters.