1)Why don't we pray ghaibana namaz e janaza (absentee funeral prayer)?2) I read somewhere that Prophet prayed it for Najashi. Is this true?

Namaz e Janaza is wajib in all but once one person has prayed it, obligation is dropped from the rest. Therefore no need for ghaibana.

Regarding Najashi some people say that he had accepted Islam and was in taqiyya