If I am not a business owner, why am I required to fill out a Schedule C form?

If you received income reported on Form 1099-NEC, then the IRS is expecting to see that income reported on Schedule C for self-employment even if there are no business expenses being claimed.  

If you receive a Form 1099-NEC from a company showing your income, the company that is paying you is not treating you as an employee.  If you were an employee, you would be receiving a W-2 and the company would be paying a portion of your Social Security and Medicare tax.  Instead, they are treating you as an independent contractor by issuing you a Form 1099-NEC and you are responsible for paying all of your Social Security and Medicare tax in the form of Self-Employment tax. 

This is accomplished by entering your income and expenses on Schedule C for your self-employed business.

For more information, see the following TurboTax article: Am I an employee or an independent contractor?