That is because they are all just playing the same character, whatever the role. Sidney Potier started the slack-faced 'dignified' look as a direct contrast to the mobile features and rolling eyes of the way blacks were previously shown, e.g. Step',n'Fetchit, "Yassuh, nossuh, lawks-a-mussy!" so they are all playing as Potier, whatever the role. It's all the same supercilious affectation of offended dignity.