Why are Sunnis and Shiites considered two different sects despite their shared practice of reading the Quran and performing Eid prayers?

Those who ascribe to Islam are divided into different sects in beliefs and worship, and the best of them are the People of the Sunnah and the Community (Ahl al-Sunnah wal-Jama'ah) who adhere to the Qur'an and Sunnah. The worst of the sects are the hypocrites who outwardly show Islam but inwardly conceal disbelief. The remaining sects fall between these two extremes. The Isma'ilis are a sect among the extremist Shi'a (Ghulat al-Rafidah); they outwardly profess loyalty to Ali (may Allah be pleased with him) but inwardly conceal disbelief. They are called "al-Batiniyyah" (the Esoteric) because of their claim that texts and laws have inner, hidden meanings that contradict what Muslims commonly understand. Their doctrine is based on secrecy, where knowledge of the true nature of their beliefs is reserved for their masters, who exploit their followers, impose financial burdens and absolute obedience upon them, and command them to oppose the People of the Sunnah.