"good afternoon" is "צהריים טובים" so why can צהריים not be translated as afternoon?
Technically, noon = צהריים, afternoon = אחר(י) הצהריים
But, saying אחר הצריים טוב in Hebrew is about as uncommon as saying "good noon" in English.
While it may be more correct to do so at certain points of the day, generally people don't.
In English one would say "good afternoon" for both noon and afternoon, and in Hebrew "צהריים טובים" for both.
In English, if it is before 12:00 pm it is good morning. At 12:01 it is good afternoon. There is a very short area where you could say good noon. I doubt it is something that is heard except in very rare instances.
Good afternoon is אחר צהריים טוב
צהריים טוביםliterally means good afternoon