You need bacteria in your gut to produce vitamin K, this takes a while. You can administer vitamin K orally and everything will usually be fine. The issue arises in rare cases where the bile duct hasn’t developed properly. There are other signs but one of them is vitamin K not absorbing properly since it’s fat soluble and you need bile to absorb fat. Administering vitamin K as a shot circumvents the gut and prevents children with these rare conditions from bleeding to death before the issue is detected.

The base rate of infant mortality used to be significantly higher than it is now. It’s possible it was always there and we didn’t evolve it away for some reason