What do “Syeds” hope to achieve by marrying only a Syed ?

One of my class fellows from High School is unmarried in his 40s because he hasn’t found a “Syed girl” yet. Also seen countless other Syeds searching Syeds because they “cannot get married out of Syeds”. This is the routine practice of a vast number of Syeds in Pak, if not majority.

Why do these Syeds restrict their marriages to Syeds only? Is it a perceived religious injunction, a superiority complex, pride, or something else? Like keeping their chance to give birth to “imam mahdi” by maintaining their pristine Syedship?