Why isn't the central Shi'a tenet of Imamah mentioned in the Qur'an as a Pillar of Islam

To be fair, even central principles underlying the normative practice of Sunni Islam are nowhere mentioned in the Qur’an. The Sunni Foundations of Fiqh (Usul al-Fiqh) - such as using the texts of the Qur’an and Hadith, employing Qiyas (analogy), Ijma (consensus), Ijtihad (reasoning) and opinion as well as the Sunni Maqasid al-Shariah (Foundations of the Religious Law) which jurists use to interpret religious law are nowhere specified in the Qur’an either.

In fact, the Formula of the 5 Pillars (arkan, da’a’im) of Sunni Islam are not mentioned in the Qur’an as a group or collection of mandatory duties. Sunnis also consider the Khilafa or Caliphate as a necessity (wajibat) for Din and Dawla - faith and world - and the Sunni concept of a Caliphate where the Caliph is selected by communal election or consultation is not mentioned anywhere in the Qur'an.

The Sunni answer to the above would be that the Sunni Caliphate, the Usul al-Fiqh, the Maqasid al-Shariah, and the Pillars of Islam can all be extracted and found in different parts of the Quran and Prophetic Hadith that are interpreted as a whole and that the general ideas for these principles are found in the Quran in the form of specific cases, from which the principle is extracted by the scholar. Now this is fine, but then the principles of Shia Islam must be seen in a similar light.

From the beginning, Shias have argued that the principle of Imamat - hereditary religious leadership that is selected by God - is found in the Qur’an and Prophetic Hadith even accepted by Sunnis. In the Qur’an, the Imamat is described through various names and titles such as:

1. God always appoints successors to a Prophet from the family members and descendants of that Prophet, often called Imams, Khalifahs, Heirs (wurathah), Descendants (dhurriyyah), Family (ahl al-bayt), and Progeny (al) in the Qur’an and these descendants of Prophets are granted both Prophethood, the Kitab (a symbol for continuous Divine Guidance) and Hukm (theoretical and practical wisdom to judge). When prior Prophets have successors (who are not always new prophets), then Prophet Muhammad should also have a successor. For example, here are specific cases mentioned in the Qur’an and the Bible contains more examples:
   1. The Qur’an in 3:33–34 emphasizes and declares a direct succession of Adam, Noah, Abraham, and Imran as a lineage of direct descendants of God’s Chosen family.
   2. Abraham’s direct desendants are given Prophethood, the Kitab (Divine Guidance), Hukm (Judgment) and the Kingdom (mulk) - see 4:54, 57:26, 29:27, 6:84–89.
   3. Thus, Abraham became a Prophet and Imam (2:124) as a direct descendant of Noah, who himself was a direct descendant of Adam (3:33–34 and Genesis).
   4. Abraham was succeeded by Ishmael and Ishmael’s direct descendants, whom the Qur’an calls “a nation submissive to God” (2:127–129, 14:35–40); Ishmael’s direct desendants include Prophet Muhammad.
   5. Abraham is also succeeded by Isaac, and Isaac by his son Jacob (21:72–73) - they are called “Imams who guide by God’s Command”; the Prophets of Israel all descend from Jacob.
   6. Moses is deputized, helped, and succeeded in leadership over Israel by his brother Aaron (20:30, 25:35, 7:142, 20:32);
   7. Moses and Aaron are succeeded by “Imams who guide by God’s Command” (32:24)
   8. Saul is selected by God as the leader and he is given leadership, knowledge, power and sovereignty (mulk) over the Israelites by God (2:247)
   9. David is appointed as the Khalifah of God in his role as judge (to give Hukm) - see 38:26.
   10. David's son Solomon inherits the authority of David (27:15–16).
   11. Zakariyyah prays for God to give him a son John to be both his heir (wali) and the heir (warith) to the House of Jacob (19:4–6).
2. Prophetic family succession is an established principle in the Quran and according to S. M. Jafri, the number of verses emphasizing the special status of a Prophet’s family numbers to over 100. Based on the Quranic model, it is not only reasonable but obvious that Prophet Muhammad’s family, his Ahl al-Bayt, holds a special and sanctified status among all the believers - and this is also consistent with the Arabian concept of Ahl al-Bayt. Thus, it would be entirely natural if not axiomatic that Prophet Muhammad appointed his successor as an Imam by Divine inspiration from among his Ahl al-Bayt just as prior Prophets like Abraham, Ishmael, Isaac, Jacob, David, and Zakariyyah had done per the Qur’an.
3. Divine Justice and Logical Necessity: Prophet Muhammad’s role was NOT to simply deliver a text-book called the Qur’an to his people and then leave them to it. This book-only model of divine guidance is NOT Qur’anic. In fact, in several places the Qur’an explicitly REJECTS the “book delivery” of Divine Guidance as something false - see 6:7, 4:153. Instead, the Qur’anic model of Divine Guidance is one where the Prophet RECITES divinely-inspired recitations (qur’ans) on a piecemeal and responsive basis and CONVEYS GUIDANCE to the community on all matters as and when they come up - see the verses 2:151, 62:2, 3:164, 16:44. The Guiding Role (hadi) of the Prophet is different from his Revelatory Role (rasul), and the former should logically remain on earth and be continued by an Imam after the Prophet.