The point, since you apparently seem to miss it, is that most of the proponents of a flat tax wish to do so to replace the current the progressive tax system that includes the plethora of exemptions. No that does not change all of the laws but it does change the progressive tax rates and it does remove the plethora of exemptions. I hold that the better system would be to eliminate income tax altogether.
All they ever emphasize is replacing the progressive nature of income tax with a flat rate. They do some hand-waving regarding cutting loopholes, but I've never heard any of them seriously attack the practice. Which of the following would the flat-taxers abolish?
The mortgage interest deduction?
Deductions for children and dependents?
Exemptions for religious groups?
Employer provided health insurance?
Earned Income tax credit?
Charitable contributions?
All of the ad-hoc 'incentives' designed to push people in to spending their money, or otherwise behaving, in ways the government dictates?
If you can find flat tax campaigns that propose to get rid of these, I'll get on board.