Probably because when people marrying their cousin isn’t incredibly frequent like European nobility, it isn’t as gigantic of an issue. Maybe they also found such marriages so rare, that they didn’t find a need to make a law about it.
Inbreeding was a huge problem within European nobility.
That is why, I presume, that with modern knowledge about genetics, Royals are marrying out.
Well, it is happening in England now, and has happened in Norway.
The genetics of inbreeding are simple. If you win the lottery it can even enhance traits. Think the crazy Targaryans in Game of Thrones. Equally likely and more deadly is the chance of two defective recessive genes combining.
Some mention the muslim world. In many countries cousin marriage is the norm. AND, for generation through generation. WHich of course quadruples the chance of a bad gene match. This is a “hidden” secret in Europe, where newborns from say, Pakistan have a lot higher rate of birth defects.
Sad all together. Do not have sex with your cousin :P
“Probably because when people marrying their cousin isn’t incredibly frequent like European nobility...”
Prince Charles comes to mind...