Actually reversals frequently happen. For example, a multi day delay is built into ACH, partly for this exact reason. Also, with credit cards and some flavors of bank transactions you as the end user are protected against fraud, even if (yes)the money has already moved on, you are made whole. The loss is absorbed by the bank. By absorbed, of course I mean you've paid for that service with various fees which amount to a kind of insurance. (Nothing really comes "for free".)

But yes, there are plenty of other types of transactions, like gift card scams, that you would not be made whole for.