Our similarities are fundamental and our differences are central.
The study of genetics has shown that our racial differences are entirely superficial. There is actually more genetic variation within racial groups than between them. The vast majority of physical differences among us are simply in the shape, color, texture, etc… of our outer coverings.
Another reason to focus on our differences is something more specific to US culture: we often have a sense that we have no culture, that we just see things the way they are and do things in the way that makes most sense, especially us Whites. This is a bit different than what many people in other groups may feel about their own cultures. People who come from cultures that are not as dominant as White US culture often have a sense of what culture is and how all of us use culture to make meaning. They may prefer their own or denigrate others’ cultures, but the concept of culture itself exists in their psyches. In White US culture, the sense that there is no culture means that we can’t even have the conversation, or that if we do, we are arguing from a stance of superiority that is greater than most. We are the enlightened ones, standing above the quaint concept of culture, showing respect for the concept because other people seem to need it while we don’t ourselves. It dampens our ability to invite cultural differences into our experiences and makes us less skilled at inviting culturally different people into our lives.