It's the same question as why people keep so many valuables in houses where you can just break a window, walk in, and take stuff: because you go to jail if you do that.
If ACH was different and you could just, say, walk up to an ATM and punch in a routing/account number and withdraw cash, it'd be a very different situation. You can only interact with the ACH system through a regular bank, and regular banks have KYC regulations that mean the recipient of those funds has a name, address, and SSN. Doing theft via ACH means the funds end up in account clearly tied to your identity, and you go to jail.
If ACH was different and you could just, say, walk up to an ATM and punch in a routing/account number and withdraw cash, it'd be a very different situation. You can only interact with the ACH system through a regular bank, and regular banks have KYC regulations that mean the recipient of those funds has a name, address, and SSN. Doing theft via ACH means the funds end up in account clearly tied to your identity, and you go to jail.