This is more of a political answer than economic: Most countries have a minimum taxable allowance. If so, you're not really being taxed on gross income as they have allowed some to be tax-free. Although, usually, the minimum taxable allowance is far too low to meet basic living costs in the country. To apply an effectively 'income profit' tax, government could peg the minimum taxable allowance to the basic cost of living (minimum rent, minimum food costs), but remember: everyone spends differently.

Trying to apply economic logic with government policy will give you a hard time, they're often allergic to well-thought policies. Income tax deters work, but is used to redistribute income to create work opportunities for the poor. Which usually results in a net positive effect for the economy.

Corporation taxes are paid because they benefit from public resources, such as infrastructure, and so effectively have an unpaid expense to the government. This is usually paid as a % of profits, rather unjustifiably.

It would be more logical to remove % taxes on income and corporations, as they're just stealing labour and risk-taking, and replace it with a charge, like a subscription fee, to the government's services. Charge will generally be calculated as: (Government spending / number of workers earning above min living cost)