Mostly because they don't know all your deductions. For most Americans they know your wages (W-2/1099) and certain deductions (mortgage interest, education expenses, and any charity big enough to actually file charitable contributions with the IRS). But there's a lot of deductions they wouldn't know about - dependents (especially qualifying adult dependents), child care expenses, charitable giving to smaller organizations like local churches, or item donations to charitable groups like the Salvation Army. For those reasons they can't just auto-compute everyone's taxes yet. But give it time. When every micro transaction is automatically logged all of your tax liability will be computed and deducted from your paycheck in advance. Hopefully not too long after that we'll turn into a post-scarcity society, or else things will get really grim.