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Generally what it is the time of the day whether morning, afternoon, evening or night. We wish everyone good morning then start the conversation, good afternoon then conversation begins and similarly with good evening. Why don't we say good night first and then start a conversation? It's again one of the time of the day.
I'm expecting other answers than just because after that we sleep that's why it is asked at last.
You are asking about what Malinowski called phatic language. It is language that serves a social function that is not conveyed by the literal meaning of the words. "Good morning" in the sense that you are considering is a greeting; "good night" is a farewell. There is nothing in the literal meaning of the phrases that indicates those different meanings: language, and particularly phatic language, is not a logical structure.
