This is because of the saying of the Prophet (peace and blessings be upon him): "The imam is appointed only to be followed, so do not differ from him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q3886
Ibn Buhaynah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) stood up after two rak'ahs in the Zuhr prayer without performing the sitting (qa'dah). When he (peace and blessings be upon him) completed the prayer, he performed two prostrations, saying the takbir in each prostration, and he performed these prostrations while sitting, before the salam; and because of his forgetting to sit for the tashahhud, the rest of the people also performed the prostrations with him (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3886
Hadith Gradingمحدثین:صحيح
Hadith Takhrij«صحيح۔ وقد تقدم كثيرا، وانظر مثلاً، رقم 3813»