Terms like the Habsburg, Angevin, Carolingian or Ottonian Empire are, or have been used by historians (regardless of the controversy on the appropriateness of the term “Empire” for some of them, especially the first two). They are used to designate polities that don't have successor states (as in the case of the “Angevin Empire/Area”), or that have been broken up in various modern states (the others). I think that the same reasoning holds true for Islamic history: at its maximal extent, the Ayyubid Sultanate controlled Syria, Egypt, and Arabia's western coast. Therefore, it has no obvious modern counterpart, just like most medieval Islamic polities. Then, of course, the idea of a “successor state” in European history can be criticised; it may create too great an impression of continuity, and foster the idea that modern nations are necessary products of the 9th/10th century. However, it is difficult to deny that modern France formed over the course of an organic and continuous process over the course of a millenium, though this process was not inevitable, and that modern France was not its only possible outcome. All in all, I would tend to say the different nomenclatures on which you remark are guided by differences in European and Near Eastern histories (I don't feel confident enough to give a reason for this, at least from the point of view of Islamic history).