Why are "good morning", "good afternoon", and "good evening" introductions but "good night" is a kind of goodbye?

Good night is seen as a farewell for a few reasons, but the biggest one is that you're implying that you'll be headed to bed. In the morning, you're just getting ready to start a new day, while good evening and good afternoon both imply that there is still time left in the day. Good night is used to signify the end of your day.

I hope this helped! If you have any other questions, just ask, and I hope you have a great day! :)

The first 2 are introducing your self, when you meet a person you say hello, good morning, good afternoon, But good night indicates that you are leaving and going to bed