They do it because it’s easy. No need to explain your setting for 50 pages
Laughs in Ascendance of a Bookworm.
It's by far the most realist depiction of it that I have seen in media, there are differences to reality as it is a work of fiction:
but the whole noble society does work on the basis of exploiting those underneath them (including among nobles) while still strenghening noble society against everyone else. And although nobles do have much better material conditions than everyone else nobility isn't viewed as good or romanticized.spoiler
there is magic and only the nobles have it which cements their power