To get a birth certificate, you must provide government photo ID with a name matching that of one of the names on the certificate you're trying to get. So you can get your own, or your child's, but not some random other person's.

Lots of people were born before RealID driver's licenses. Some of them went by names other than the names on their birth certificates, and thus are unable to get new copies of their birth certificates using the government-issued photo ID they currently have. E.g. I've got a grandfather who went by Sam his entire life but was apparently named Harold. His driver's license had Sam as his first name. If he had lost his birth certificate, he would not have been able to obtain a new copy legally using that driver's license! This still happens to people. Also sometimes house fires or similar disasters happen, and people lack the ID needed to get new government-issued ID.