Why do I have to fill out Form 8615? I am a full-time college student filing independently.

It's possible to not be a dependent and still be subject to the kiddie tax.  To avoid it, you must be supporting your self by working (earned income).

Who Must File for 8615
Form 8615 must be filed for any child who meets all of the following
conditions.
1. The child had more than $2,200 of unearned income.
2. The child is required to file a tax return.

3. The child either:
a. Was under age 18 at the end of 2020,
b. Was age 18 at the end of 2020 and didn’t have earned
income that was more than half of the child's support, or
c. Was a full-time student at least age 19 and under age 24 at
the end of 2020 and didn’t have earned income that was more than
half of the child's support.
(Earned income  & Support are defined in the reference below)
4. At least one of the child's parents was alive at the end of
2020.
5. The child doesn’t file a joint return for 2020.
For these rules, the term “child” includes a legally adopted child
and a stepchild. These rules apply whether or not the child is a
dependent. These rules don’t apply if neither of the child’s parents
were living at the end of the year.

Reference (instructions for form 8615): https://www.irs.gov/pub/irs-pdf/i8615.pdf