there are lots of more interesting civs that come from this area that would make better launch material due to campaign potential. take the Umayyads, who were pivotal in the golden age of the Cordoba Caliphate and played major roles in the conquests of Persia, Rome, central Asia, northern Africa, and Iberia, and had at one point the strongest navy in the Middle Ages stretching unrivaled from the Mediterranean into the Indian ocean. or the Turks, who had major role throughout ad past the Middle Ages, whether it be the crusades, the roman campaigns, the Mongol invasions, and their central Asian/northern Indian dominance. Even the Fatimids would be better campaign wise, with their unique blend of Berber, Sudanese and Turkish troops and their major involvement in the crusades, allying with and enabling the first crusade to succeed through the constant captures/loss of Jerusalem and acting as the main opponents of the Suni Sultanates.
Meanwhile the Abbasids are one of the least interesting civs from the region, as they had essentially no territorial expansions and most of their conflicts where either border skirmishes or civil war. keep in mind that this is a dynasty that through incompetence and what can only be described as a shocking indifference to self-preservation not only lost all the territory they inherited in their revolution but had done so mostly through their own inability to govern rather than through outside influence.
they were essentially puppets under house arrest for almost all their reign, with no territorial accomplishments or campaign worthy events to speak off. the navy was left to rot, the army was purged of its most competent officials for fear of rebellion, then replaced with slave troops who rebelled anyway, and even in the Mongol campaign they were little more than a side note, with a minor engagement that was a crushing defeat before the capitol was sacked and any pretense to power removed. They spent most of their dynasty staving off sieges of their capitol from their vassals rather than having any sort of influence or projection of power, not exactly campaign worthy material.
they did have cultural and societal achievements, such as removing the ethnic and cultural restrictions of the previous Umayyad caliphate in favor of espousing racial equality and the egalitarian ideals of the earlier Rashidun caliphate, if only in lip service. and their patronage of the arts and sciences is well documented, along with the spread of paper making to the wider world. but age of empires as a series has always shied away from the dark side of human history, such as slavery and racism. And when your crowning achievements are emancipation, tax reforms and culture, and a complete failure at everything else related to running an empire, it is hard to think how they could be integrated into interesting campaigns. there is really nothing in the way of heroic defense or world-shaking conquests, and that is what makes it a shame they are included as a launch civilization.
And the main reasons you would include the Abbasids are not even in the game. While the culture/science aspect seems to be represented with the fictional house of wisdom, there is nothing we have seen so far that references their multicultural policies and tax reforms, which were the primary defining feature of the Abbasids and the driving force behind their revolution and continued existence/tolerance from the breakaway Suni states. Not even the unique units they had in history are represented, such as the naffatuns, instead we get fantasy units such as camel archers.