Because our political divide is based around education levels
Educational levels are more polarizing than ethnicity. Well-educated people come from very different cultures and, even if they don't intermarry much, they recognize the validity of the cultural differences that they still have. Getting a degree from UC Berkeley in chemistry can give people from antipodal places (let us say Shanghai and Buenos Aires) much in common.
This said, white people from the wilderness areas of central Pennsylvania can have nothing in common with people from suburban Philadelphia, which could be about a two-hour drive. (There are few non-white people in Appalachia, so that is the more relevant distinction). Religion is likely different, and so are educational expectations.
