I always thought the obvious reason for this is that it allows the banks to hang on to your money a little bit longer. At least that's how it works where I live

I always thought the obvious reason for this is that it allows the banks to hang on to your money a little bit longer. At least that's how it works where I live. The money disappears from your account immediately, so essentially you give them a ~24 hour free loan. The banks win, so they have no incentive the change it.