I think there needs to be a distinction between “demographic realism” and “psychological realism” (and other ways that a work can be more or less realistic).

George RR Martin wasn’t interested in creating a plausible world, but he was interested in having the characters within that world act plausibly given the circumstances the world gets them; the end result is that Westeros has highly improbable demographics, history, etc, for a region of the size it’s supposed to be, but has characters who have understandable and complex motivations rather than just being a Hero or a Villain.

Throw in an almost complete lack of high-fantasy elements and you have a series which is “realistic” enough to interest people on that basis despite lacking realism in certain areas.