domestic EFTs actually happen within 15 minutes (up to an hour for SWIFT), and get posted the next morning.

interbank transaction clearing (at least in Canada) runs from 10pm Eastern each night, and final posting must happen before 8am the next day, or there are steep penalties. like fines to CPA/OSFI plus loan interest to the other party on that $18 Billion you were late delivering in the overnight clearing cycle.

the banks advertise a 3 day clearing for consumers partly in order to protect themselves from backdraft if some outsourced InfoSys doofus intern nukes a file transfer and they miss the daily clearing window to balance with their cronies (rarely happens b/c the penalties are so high).

but mostly it's to harvest 2 days worth of profit for themselves either at the interbank interest rate or (depending on how strong the max VaR is on their balance sheet) floated in the markets, used by their HFT for even more profit-at-your-expense, then put back.

all the while, also being used to ghost their faked BASEL capitalization requirements.

that's why.

TL;DR: it's a legalized man-in-the-middle attack by the banks on your money while it's in transit, that you can't do anything about, because you're just a peon like the rest of us.

GLAL,

R.