Ask HN: Why do automated bank transfers take X "business" days?

I'm fighting with a transfer to an Amazon account from my bank, and it frustrates the hell out of me that it takes "5-7 business days". I initiated the transfer almost a week ago, and it's almost 5pm on a Friday, meaning it probably won't complete until next week.

Ok, I can understand that it has to take a little time, but the "A" in ACH stands for Automated. Why does a computer care whether it's Tuesday at 9am or Sunday at 1130pm? Execute the transfer and mark it complete. Case closed.

It just seems ridiculous to me that automated transfers are restricted by business days/hours, and the overall slow speed in general irritates the crap out of me.