Why did the Fatimids reject the Abbasid caliphs of Baghdad?

The Fatimids rejected the Abbasid caliphs of Baghdad because they viewed them as illegitimate usurpers.1

Unlike other regional rulers who acknowledged the Abbasids' token authority, the Fatimids were leaders of the Ismāʿīlī sect of Shīʿī Islam. They claimed descent from the Prophet Muhammad through his daughter Fāṭimah and his kinsman ʿAlī. Consequently, the Fatimids believed they were the rightful imāms and caliphs, chosen by divine authority to lead the universal Islamic community. Their objective was not merely regional independence but the total overthrow of the existing religious and political order to establish their own caliphate in place of the Abbasids.1