The main service provided by banks is that they sell trust. Only banks and financial institutions can directly send ACH transfers. Unlike wires, ACH transfers

The main service provided by banks is that they sell trust. Only banks and financial institutions can directly send ACH transfers. Unlike wires, ACH transfers take time to settle. Banks are tasked with only allowing authorized transfers. Any bank that does not take adequate precautions will be kicked out of the system. The profit banks can draw from participating in the financial system is much higher than the amount they can steal once before they get banned. That's why most banks ask for verification (logins via plaid, micro deposits, etc), before allowing ACH withdrawal requests from other accounts. But the most important reason why people don't do this, is that it's very easy to get caught. To actually submit an ACH request to a random account, you have to appear somewhere in person (unless you do small checks up to your daily bank limit via picture, which will quickly have you caught since your phone and account is linked to a real person). Some CCTV tapes later, and you'll be put in jail for a very long time.