The question is, how do you calculate income? What deductions are you allowed?
A regular income-earning individual cannot take more than a fixed amount of deductions (in the sense his tax rate is fixed). However, a small business owner or self-employed invididual can deduct business expenses from his income. And there in lies the problem. How do you classify expenses? This creates an incentive to find creative ways of deducting your taxable income, so much that there is an entire industry formed around it.
A regular income-earning individual cannot take more than a fixed amount of deductions (in the sense his tax rate is fixed). However, a small business owner or self-employed invididual can deduct business expenses from his income. And there in lies the problem. How do you classify expenses? This creates an incentive to find creative ways of deducting your taxable income, so much that there is an entire industry formed around it.