system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: ACH Deposits: Why do they take so long?

It's all digital. It's all through the internet. When I send money via ACH, it's almost instant, right? So why is it, when I'm expecting a deposit and the institution says they sent it on a certain date, it takes "up to 5 business days" for it to appear in my account? Where is the money for those "up to" 5 business days? I just cannot wrap my head around this, and quite frankly, it irritates me. It's all binary code, it's just bytes of data, but those bytes of data are what make the world go 'round.
gpt
ACH is very old. It was built on the backs of the first computers, long before the internet allowed for instant communication. ACH is built such that there are manual steps in the transaction process. It used to be that banks would compile the days ACH transfers onto tapes once per day, and ship the tapes to the clearing house to have the transfers distributed to the banks they are meant to go to. The clearing house was operated by people on business days only.

In the US, ACH is not controlled by the government, but rather by an organization called NACHA that is made of representatives from most banks in the US. A few years ago, NACHA held a vote to upgrade ACH and make Same Day ACH the default. It got a majority vote in favor, including vote from the big banks. It failed, though, because it needed a supermajority to pass, and many small banks voted against it because they do not have the money nor the tech to actually support the upgrade.

There is a new system, Real Time Payments (RTP) that is slowly gaining support. But this system currently requires an even larger technical investment to support, so only the largest or most technically advanced banks have it.