If all I make is 8000 dollars of self employed income, why do I have to pay taxes anyway (less than standard deduction)?

If the $8,000 of income were wages, you would have paid FICA taxes on the wages. 

Self-employment income of over $400 is taxed for employment taxes...e.g., Social Security and Medicare.

You shouldn't be paying any income taxes.

best and most complete answer....it's not really income tax....it's the IRS collecting social security and medicare taxes.  Only the poorest of the poor self employed can escape being taxed somehow (ie, <$400)

when Self-employed you can't escape the SE tax of 15 %.

better answer was "you're paying some medicare and social security taxes, really....not income tax" on ANY money over $400 in Self Employed income.