I always just assume it so they earn interest on the money they have withdrawn immediately from the payer's account, yet refuse to deposit into the receiver's account for 3-5 biz days. I don't think I'm being cynical about it. It's the only logical explanation. Computers only working banker's hours is not a logical excuse (while fun to joke about). A phone support person tried to tell me that a human still approves the transactions. Whatever that means, but it's not logical either.
Interest rates have been effectively 0% since the 90s.
> A phone support person tried to tell me that a human still approves the transactions.
This is true in some cases. They do at least have to audit some of them manually and possibly close your account, file SARs, etc, if they don't like you enough.
They're just making sure they can avoid needing to pay their computers overtime when they get worker's rights. /s