To make things simpler, ironically.The purpose of the AMT is to ensure that the uber rich pay at least a minimum amount of taxes, but has since morphed into something that hits the upper middle classes*. It does that by having fewer tax brackets, fewer allowed deductions and a higher standard deduction. What you owe is whatever causes you to pay more taxes.However, this needs to be done in addition to the traditional tax calculation. So you need to take your calculations of your various income measures, and put back in various deductions that are disallowed under AMT rules. Or have to be recalculated. Itu2019s a pain.Either someone decided that this was easier than having a completely separate tax form to calculate your AMt tax or someone lobbied to have mor complicated taxes so youu2019d go to one of the tax places or download tax software.*With the Trump tax changes, AMT affects fewer people.