That means you need engineers on call 24/7/365, which raises the cost. One of the nice things about ACH is that it's dirt cheap, especially compared to the credit card network.

For example, Stripe charges 0.80% for an ACH transaction, capped at $5 max. Some processors just charge a flat rate. A credit transaction is a whopping 2.90% + $0.30, with no cap. When it comes to my paycheck and bills, slow and cheap is totally fine.

On top of that, many banks don't even post transactions on weekends/holidays anyway. They'll remain pending until close of business on the next weekday. That's been starting to change, but still.

The banking system is built around store-and-forward and batch processing, with lots of auditing in between. This is a feature, not a bug. :) Sure, it could be speed up... but if you really need instant, there are already options that exist (debit, credit, wire). You just have to pay for them.

(Fun fact: Even the credit card system isn't really instant. The credit authorization goes through immediately, but the actual charge still takes 1-2 days to clear... and is often done in end-of-day batches. And yes, there are exceptions... but it's not the norm.)