It's not racism per se but the tyranny of normalcy - no: the tyranny of attractive normalcy. Which leads to loveable white models who are supposed to be playing ordinary, adorably flawed professionals just like you and me with their brilliant minority friends (with vastly less camera time) who are surgeons. But it's not just ethnicity. That narrow vision also extends to, say, things like women leads. Women leads have to be good-hearted and nice, with a Slutty Best Friend. The main character can't be slutty. Because that's not attractively normal etc
