Why do some people care so much about racial purity?

The idea of racial purity is a myth. We are all related (being the same species) and just because my physical characteristics are similar to yours, doesn't mean I'm more closely related to you than say, an indigenous Amazonian villager. Genetically I may have more in common with the Amazonian than I do with you. We are all swimming in a complex gene pool and those people that prize superficial physical similarities over hard genetic facts are deluded.