How come the term 'good night' isn't used as a greeting, but only as a...

...farewell/goodbye? You tell someone good night and it's usually when you're saying bye. I never hear it when you're saying hello, as others are used: "Good Afternoon" "Good Morning" " Good Evening"

I guess "Good Evening" is the alternative to "Good Night" but evening, to me, is like 5pm to about 7 or 8 and night follows that.
 
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