The standard progressive view seems to be that the stereotypes were based on the fundamental attribution error: attributing negative traits that a group has to their innate nature rather than to negative influences.

The problem is that progressives consider it evil to attribute negative traits to innate nature and will refuse to update in that direction even if changing influences doesn't improve the trait.

Thus if a negative trait happens to actually have an innate cause, progressives end up going on witch hunts trying to find the w̶i̶t̶c̶h̶ oppressor whose evil s̶p̶e̶l̶l̶s micro-aggressions are causing the negative influences.