Since no one has answered I'll take a crack at this.

Under the Seljuqs marriage does seem to have been way of securing power or allegiance. Nizam al-Mulk married two of his daughters to Shia notables for the sake of stability. Alp Arslan married Toghrul Beg's widow. Wikipedia lists a number of intermarriages with the Georgian Bagrationi dynasty including Nizam al-Mulk himself.

So to a limited extent it did happen.

But one of the defining features of both these medieval dynasties and of the lower titles that existed under their rule in the Islamic world is that they weren't really hereditary dynasties at all. The iqta, at least in theory, would not pass on to your children so you don't end up with a sophisticated noble class like you do in Europe. In Egypt by the time of late Fatimid caliphs (and in a system perfected by the mamlukes) the Sultan's successor is completely non-hereditary. In the east this is completed somewhat sooner as the caliph cedes political/military authority to the Amir al Umara.