“Hi” here is what’s called an interjection. Sort of like if you were trying to suddenly stop your friend George from doing something and you said “No, George!” “No” would be the interjection. In the same way you would say “No, George!” instead of “No George!” (which reads like you’re protesting the existence of George), you would be technically correct in saying “Hi, George!” However, nobody would seriously care outside a grammar class if you said “Hi George!”