Filmmakers are partly responsible for reinforcing these standards, and audience (general public) are also responsible for holding these standards/views. Films are representation of fantasy men have. An average man want woman who looks like the ones shown in films, and since he can't get that, he chooses to watch films and live vicariously. Filmmakers don't have much conscience, not even the so called socially responsible filmmakers to caste actresses who don't match the fantasy average men has.. Films are business and if "conventionally attractive" women get more audience, filmmakers will go for it. If this trend changes and people stopped caring about skin tone, or prefer caramel skin women, then filmmakers will cast such women. It is all about profit at the end of the day.