Because the IRS doesn't know how much money you owe. They know approximately what you made, and they know a little bit about some of your deductions, but they don't know whether and to what extent you are entitled to additional deductions or credits, or whether and to what extent you earned money from transactions not reported to the IRS. Even on the transactions that were reported to the IRS, the IRS doesn't always know how much of that income is actually taxable - or at what rate.