Why can't everyone except that we are all one race?

The way I see it is that we are all under one race. The human race! and we keep this up. we will advance are technology and the earth will be poisoned from all of our bio-hazards that we put on this earth. Look, we all have are skepticism and belief but why not join together to create harmony? No Im not a tree huger, but this is one of many questions that are on my mind. Feel free to express.