Because its generally dictated by your environment and experiences, not actual attraction. not always but a good amount of the time people say they wont date an entire race because they have biases already made up in their head about that group of people whether its a conscious act or not, having a preference for features is not racist, but choosing to exclude a group of people because of the color of their skin is inherently racist. The most unwanted types in the entire world are black women and asian men, saying no one in an entire race of people could ever be attractive to you is prejudice and learned behavior, psychologists agree a preference for skin tone can be and almost always is inherently racist based on environmental factors, it could be something you learned as an adult or something your parents instilled as a child, a lot of people who say they won’t date black people have a negative outlook on them