Old movies were made by people who actually went outside and knew what the real world was like. If once upon a time in the west was made today nobody would have tan skin, blemishes, balding or wrinkles, nobody would sweat, their clothes would just have some mud randomly slapped on instead of being realistically tattered, and they'd all have weird builds because their only exercise comes from a gym(or they get none at all).

Oh and ofc there would be no pretty landscape shots or well composed scenes like pic related, it'd just be closeups of people talking or shooting each other and nothing else