For a set of reasons:

  1. Mating selection is determined by both genetic markers (physical properties) and status signals (social properties).
  2. There are differences in desirability between the races due to different morphological attributes, despite the near universal human preference for a set of attributes. 
  3. There are different DISTRIBUTIONS of certain talents across the races. (linguistic intelligence, and spatial intelligence in particular.) This difference in distributions causes the development of different norms and preferences within groups, which in turn alters the complex signals we both observe and send.
  4. Because of this economy of signaling, Status Signals ‘within group’ are lower cost than status signals ‘across groups’. (Partly because we have just have higher familiarity within the group). Each of us is more likely to get more positive, and fewer negative status signals within group than across groups. And those signals are richer and more complex.
  5. These signals affect our relationships and the trust that can develop in them.  Where that trust is necessary for relaxed interaction, goal determination, task coordination, and risk taking.
  6. In the working and lower classes, external racial groups usually will work for less money or will displace them in their earning capacity and therefore also deprive them of status signals.  Racism is a means of forming political solidarity themselves, as well as with their elites, for the purpose of preserving their advantage – or gaining their advantage.
  7. In the middle and upper middle classes, racism is a vehicle for maintaining political power (law) and social power (norms) and assets (their own accumulated status signals) for themselves and their groups. This set of facts is demonstrated by our demonstrated universal preference to work (largely) and live (largely) with people who share our same ethnicity and social class. The data illustrates that preference over and over again.  In simple terms, we are ‘judged’ more easily, and therefore included more easily among those with whom we share physical, intuitive, conceptual, and habitual similarities. However, at the extremes, the very successful and prosperous tend to form a worldwide-class and the lower classes seek mates more opportunistically, and there are social signaling benefits to certain racial groups (a mating between a below average white woman and an above average black man may increase the social standing and quality of mates of both. So the racial norm is a majority-middle preference. 

While there is a noticeable rise in the inbreeding going on between asians and whites,  women still seem to demonstrate an extraordinary preference for men within their race (men are less discriminating) of upwards of 80%.  But this preferences is a middle class statistic obtained from dating sites. And it becomes very hard to make the same statements about the lower classes outside of what’s stated in the census (about 15% intermarriage).  The reason is that some races are pretty indistinct (black/hispanic) because of high interbreeding already.

I hope this was helpful. This is only a sketch of a complex topic. But it’s enough of a trail of bread crumbs that it might help answer your question.

https://www.quora.com/Why-does-racism-exist-and-from-where-did-it-originate