Why do the syed families not marry their daughters into a non-syed family?

Because they don't know the religion! Actually they are not really "syed"[1]. It is a false claim. They are caught in the caste system of the Hindu religion from which they came. They cannot trace their lineage authentically to the descendants of the Prophet (ﷺ), and even if they could, Abu Taalib, who was the Prophet's own uncle, and Abu Lahab another of his uncles are both in the Hellfire!

So what is the value of being one of their descendants?

Reference:

[1] A syed is a descendant of the Prophet (ﷺ) , aka, ahlu bayyit and in certain countries as Habib for a male and Shareefah for a female.