I presume OP is asking about the United States and there are a lot of great answers about that. But as a fun fact many other systems work completely differently. Europe is now implementing instant transfers (under 10 seconds), any time, any day of the week, even for international transfers, for free. So I’d say the real reason is that banks resist innovation and change.
[https://www.europeanpaymentscouncil.eu/what-we-do/sepa-instant-credit-transfer](https://www.europeanpaymentscouncil.eu/what-we-do/sepa-instant-credit-transfer)
edit: Thanks for the replies below. It’s good hearing about how thing work and how they’re improving all around the world.
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