The tax law was once simple, until people found a loophole and then another and then another and then another and so on.

If you say X% on realized income is that a tax on sales? Someone starts to manipulate sales price via incentives and adjustments to not pay tax on it. If it is net income now you have to write rules about what deductions are allowed against the gross receipts.

For individuals, is it X% of wages? What about self-employed? Is it gross receipts or net income? Now you're back to the issues above.

This is how it got quickly complicated after starting much more simple.