Why would I want both a less accurate book keeper and to incur all of the liability of doing the books myself?!
Presumably your current book keeper is not your slave, and you have to pay them...
Because it's way cheaper. Not saying it's a good tradeoff, but that appears to be their pitch.
It's not less accurate. As commented above, the control knew they were being tested against the machine, so made sure to be super careful.
In everyday life the human is less careful, and the machine costs 1% of the human.