Because we aren't different species - just a variation.
We're all humans.
A black tom cat can have kittens by a white queen cat. A ginger tom can have kittens by a tabby. And all those kittens will be able to reproduce.
Because different "races" of humans are genetically close enough to be able to reproduce without complications. There are only slight differences between Africans, Europeans, Asians, and so on, while there are more differences between horses, giraffes, donkeys, etc.