the Rothschild's is one of 2 of the most wealthy families in the world and to keep their wealth in the family they married in the family and had kids in the family...it wouldnt be unrealistic for the uchiha or senju or uzumaki or hyuuga any other clan to marry in the family to ensure clan specific jutsu/kg/pure blood stays in the family, now i believe in modern times there are some that chose to wed outside of the clan which is why u have a tsunade and in naruto's case it would leave kushina no choice but to marry outside of the family because she was basically the last or one of the last of the uzumaki
if none of the noble clans didnt incest then most of the younger generation wouldnt have the kgs of the clan since they would be watered down after years of ethnic mixing, the fact that everyone in a clan is born with these kg's or access to these kg's over time should let u knw they were some weird and nasty mutha ****as and that they we'er incestuous...