Because you have to pay self-employment tax for the consulting work. You pay SE tax for even as little as $400 of self-employment work.
You probably don't owe any regular income tax on your income but you owe self employment tax on it.
Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare.
SE tax is on the 1040 Schedule 2 line 4 which goes to 1040 line 23.