Because when the Prophet ( صلی اللہ علیہ وسلم ) prostrated, the Muslims also prostrated with him, and all the polytheists except for two fame-seeking individuals also prostrated. Hazrat Zaid bin Thabit (may Allah be pleased with him) recited Surah Najm to the Prophet ( صلی اللہ علیہ وسلم ) and did not prostrate, and the Prophet ( صلی اللہ علیہ وسلم ) did not order him to prostrate. If the prostration of recitation had been obligatory, the Prophet ( صلی اللہ علیہ وسلم ) would have ordered him to prostrate. And if there was no prostration in Surah Najm, as some people have misunderstood due to the reason of a certain hadith which we will soon mention (InshaAllah), then the Prophet ( صلی اللہ علیہ وسلم ) would not have prostrated in Surah Najm.

Hazrat Kharijah bin Zaid bin Thabit narrates from his esteemed father that he said: I recited «‏‏‏‏سورۃ النجم» ‏‏‏‏ to the Messenger of Allah (peace be upon him), but none of us performed the prostration (sajdah). Janab Abu Dharr says: I offered prayer behind Hazrat Umar bin Abdul Aziz and Hazrat Abu Bakr bin Hazm (may Allah have mercy on them), and neither of them performed the prostration of recitation (sajdah tilawah) either.