While this is true a lot of banks would like to do it instantly as it would be an advantage over others and drive more people to sign up for the feature.
The real problem is most banks backend systems are still old mainframes where this isn't possible.
Source: I'm a prior developer at multi-billion dollar payment processor working with many acquirers.
In the UK at least the answer was government fiat.
Faster Payments is the unimaginative name for the rule that allows most UK bank account holders to move money the same day (typically in reality instantly) at zero cost to them. The date for Faster Payments becoming possible was set, and banks are just obliged to provide it. Some were earlier, most were not.
The banks did not actually implement the underlying backend transfers in time, but customers don't care. Rick Smith, father to an 18 year old daughter who seemingly always needs another few hundred quid for something wants to send Beth £750 right now, and Beth wants to be able to spend that money when she receives it from her father. Neither of them cares that Rick's and Beth's banks are running different versions of some 1970s COBOL application or are struggling to ensure a backend funds transfer matching the Rick-> Beth transaction happens in a timely fashion.
So the banks just faked the UX. This technically means if Rick's bank fails after Rick sent the money, but before the backend catches up in a day or two, Beth's bank (but not Beth or Rick) could lose the value of the transaction because the underlying money actually didn't go anywhere yet, just the two account balances were updated. But regulators reasoned that banks being more likely to freak out and report if they suspect their competitors are struggling and likely to fail imminently is a good thing so let them take that risk.
Maybe they have subsequently fixed their backends, maybe they didn't, as an end user I needn't care so I paid no further attention.
