In the United States, and many other nations, the vast majority of taxes aren't collected from payroll. If that was the case, things would be easy. But there are hundreds of other ways that taxes are collected, and they vary from person to person.

More importantly, taxes aren't exclusively about revenue generation. They're used to encourage people to do or not do things: Save money, invest money, buy houses, have babies, buy cars, add solar panels, and thousands of other things.

There are plenty of people who shout "flat tax!" whenever this topic comes up. But they're mostly people who have only led simple lives where the IRS only touches them on their paycheck, or who don't understand the full range of how taxes are used.