Copying your description to Google

• first result is NIH dreck that reads like an anti-vax conspiracy. It’s presented as scientific but not worded that way. RIP trust in government.

https://kidshealth.org/en/parents/hepb-vaccine.html

If a newborn's mother carries the hepatitis B virus in her blood, the baby must get the vaccine within 12 hours after birth.

I imagine part of the reason is timing. The vaccine is safe and effective for infants so no reason not to. Meanwhile the risk of missing one is too high, that you will get a valid test back and make the decision in time to save the infant.

There wasn’t any info on costs or reliability but it’s quite likely the test is more expensive than the vaccine and not 100%. Given the vaccine is safe and effective to use with infants, timing is critical for the ones at risk, and the test adds cost with no value perhaps it just reduces healthcare costs