Maybe you guys don't have an income tax? In the US "they" don't calculate my tax. It is up to the me to report all my income, calculate how much tax I owe and then pay it. "They" give us all the forms to fill out but it is up to us to fill out the forms. How they track your income for tax evasion charges is up to the government. They have many ways to do this. Sure you can evade paying the taxes until you get caught.
Perhaps this is part of the confusion? I know many (most?) countries do the calculations and hand over a bill to the taxpayer. In the U.S. we have an unusual system that has the taxpayer prepare their own return. So if you are not used to doing your own taxes it could get quite complicated.