I have a 1099-NEC for a paid internship as part of my undergraduate degree. I am still claimed as a dependent on my parents' tax return. Why do I need a form 8615?

You do not need to file Form 8615 if the internship is your only income. You are right. The internship is earned income. You may have entered it incorrectly in TurboTax.

In some versions, TurboTax will move a 1099-NEC to "Other Income," which is not considered earned income if you say you did not have expenses. Go back to your 1099-NEC and tell TurboTax you had expenses. TurboTax will ask whether you want to upgrade to TurboTax self-employed. Upgrade. This is the only online version that handles self-employment.

You can claim expenses, but you don't have to.

The ultimate guide to tax deductions for the self-employed

If the amount on your 1099-NEC is not considered earned income it might be due to your not listing the income as income from a business on Sch C.