They blindly followed their forefathers

They blindly followed their forefathers and did shirk without any evidence from Allāh

So do not be in doubt, [O Muhammad], as to what these [polytheists] are worshipping. They worship not except as their fathers worshipped before. And indeed, We will give them their share undiminished.

Qur'ān 11:109



قال أبو جعفر : يقول تعالى ذكره لنبيه محمد صلى الله عليه وسلم: فلا تَك في شك ، يا محمد ، مما يعبد هؤلاء المشركون من قومك من الآلهة والأصنام، أنه ضلالٌ وباطلٌ ، وأنه بالله شركٌ ، (ما يعبدون إلا كما يعبد آباؤهم من قبل)، يقول: إلا كعبادة آبائهم ، من قبل عبادتهم لها. يُخبر تعالى ذكره أنهم لم يعبدُوا ما عبدوا من الأوثان إلا اتباعًا منهم منهاج آبائهم، واقتفاءً منهم آثارهم في عبادتهموها، لا عن أمر الله إياهم بذلك، ولا بحجة تبيَّنوها توجب عليهم عبادتها.

Imām al-Tabari writes that Allāh has told His Prophet Muhammad sallallaahu alayhi wa sallam to not be in any doubt that the idols the polytheists, in his nation, worship are bātil and misguidance and their doing so is shirk.

They worship these idols like their fathers of old. Allāh did not order them to worship these idols. Instead, they follow the manhaj of their forefathers. They have no evidence for worshipping these idols. They blindly follow what their forefathers did.

Reference:
Tafsīr al-Tabari