The same reason I, and millions of other buyers of firearms, do.
"I certify that my answers in Section B are true, correct, and complete. I have read and understand the Notices, Instructions, and Definitions on ATF Form 4473. I understand that answering “yes” to question 21.a. if I am not the actual transferee/buyer is a crime punishable as a felony under Federal law, and may also violate State and/or local law. I understand that a person who answers “yes” to any of the questions 21.b. through 21.m. is prohibited from receiving or possessing a firearm. I understand that a person who answers “yes” to question 21.n.1. is prohibited from receiving or possessing a firearm, unless the person answers “yes” to question 21.n.2. and provides the documentation required in 26.d.
I also understand that making any false oral or written statement, or exhibiting any false or misrepresented identification with respect to this transaction, is a crime punishable as a felony under Federal law, and may also violate State and/or local law."