It’s not. But without some sort of id, and with mothers lying about everything from their names to their ages, you can’t just start handing out kids to whoever asks for one. They HAD to do the testing, or the next complaint would have been from mothers whose kids were given to someone else.
Heck, if we did it your way, all you’d have to do if you wanted a free kid is to put on a “prairie” dress and go ask for one.
But there is a law that all children by the age of 1 must have a social security number - thus the need of a birth certificate.