They already do that -- if you calculate your taxes wrong, they will send the adjustment (they will do it both ways, pay you back or ask for the remainder). I guess they might not be aware of all the deductions, but standard deduction beats itemized one for the majority, so they can 100% automate this whole process if they decide to. For complex cases and businesses, sure, you are on your own, but at least most W2 should be covered.