Prejudice is another word for generalization. We all make generalizations, adopt “stereotypes” as working models, because they work, give a generally accurate picture of the real world and they allows us to make useful decisions.
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.