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.