Because that is how Congress wrote the law. There are no provisions for your situation in the regulations.
https://www.irs.gov/pub/irs-pdf/i8615.pdf
Who Must File
Form 8615 must be filed for anyone who meets all of the
following conditions.
1. You had more than $2,100 of unearned income.
2. You are required to file a tax return.
3. You were either:
a. Under age 18 at the end of 2018,
b. Age 18 at the end of 2018 and didn’t have earned income
that was more than half of your support, or
c. A full-time student at least age 19 and under age 24 at the
end of 2018 and didn’t have earned income that was more than
half of your support.
(Earned income is defined later. Support is defined below.)
4. At least one of your parents was alive at the end of 2018.
5. You don’t file a joint return for 2018.
These rules apply if you’re legally adopted and a stepchild.
These rules also apply whether or not you’re a dependent.
These rules don't apply if neither of your parents were living at
the end of the year.
Lines A and B
If your parents were married to each other and filed a joint return,
enter the name and social security number (SSN) of your parent
who is listed first on the joint return.
If your parents were married but filed separate returns, enter
the name and SSN of your parent who had the higher taxable
income. If you don’t know which parent had the higher taxable
income, see Pub. 929.
If your parents were unmarried, treated as unmarried for
federal income tax purposes, or separated by either a divorce or
separate maintenance decree, enter the name and SSN of the
parent with whom you resided for the greater part of the calendar
year. See the definition of custodial parent in Pub. 501.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
