And then some deride those "stereotypes" precisely BECAUSE they paint an accurate picture, but a picture which violates the impression that is to be promoted.
In the case of the Left, the promoted narrative is that we are all a product of our environments, that there is no genetic component to who we are. The reason for this narrative is that one of the Left's articles of faith is that humans are perfectible, that given the right environment and education anyone can be turned into the Perfect Leftist Person.
Great article dissecting "The Narrative" and dweebs like Rich Lowry.
"The public scolding that National Review and other mainstream conservatives bestowed on John Derbyshire is absolutely despicable. They value the Narrative over honest debate and inquiry into whether or not Derbyshire's article was factually correct or not."