I don't think IRS has all the data in one place. E.g. do they have my local property taxes? Do they have record of mileage that I could deduct for charity purposes? Do they have a value of goods I donated to charity? Hypothetical examples, there can be a lot more options, many of them not so easy for them to verify. That's why they audit - then if I am audited, I'd have to prove them it's true.
