> The US has a unified payment system (ACH) and has had it for many decades it's just not instant

And for instantaneous transfers, we have Fedwire. The original real-time wire service. It’s free for most commercial and high net worth users, and thus, from a bulk economic perspective, sufficient for industry.

Interac (in Canada) is trivially simple to use, and quick.

1. Send the payment through your logged in session at the bank or credit union, or via their respective phone app. 2. Select the recipient, adding a new one if needed. They'll need a mobile # or email address. 3. Include the question and its one-word answer that the recipient has to answer to accept. 4. Hit send 5. Recipient is notified, follows the link, and answers the question. If they have it set up, it might be auto-deposited without the question/answer.

It takes minutes, and doesn't ask the sender for the recipient's banking details.

So why isn't ACH used widespread?

Because the Fed sits on your money for 1-5 days so they can play with the float. It means you either need to wait out your transactions for this random length of time or take the other guy at his word that he send the money (LOL).