Because the hard part is to make sense of a box full of unsorted scraps of paper, some of them with barely legible handwriting on them. Much of accountancy is the process of turning such boxes into nice rows of numbers. Once you have the numbers, the arithmetic is trivial.
Seems like an easy job for AI. Take all scraps of paper out of box, record a video of all scraps, AI make sense of the handwriting and other things. Eventually make a robo that allows you to dumb the scraps into an accounting box that does all of this automatically - fish out receipts, scan, OCR, understand meaning, do arithmetic, done.
Those kinds of systems already exist. They tend to be a bit unreliable and still require a human person to oversee the process. Besides, in truth, they only handle a fraction of what accountants actually deal with.
