Humans are divided into groups based on culture, and a majority of people are loyal to their group. Given this stuation, people have to look at other people as belonging to a group. Of course, I'm not saying there are real genetic differences among races and nationalities, but there are cultural ones, and some cultures can't exist well with each other.
Ultimately, the people of the world cannot be one mainly cause of culture, not genetics. I don't know if there is a solution to this problem, but at least since the 1960s there has been massive progress in the US and other western nations.
Racists are trying to say there are differences beyond stuff like resistance to diabetes etc.. In other words, they venture into study of IQ and other ridiculous garbage, all with the main goal of trying to prove one race is superior, while of course massively ignoring environmental factors.