That's why you support more of your genes in unmixed, than in the mixed child.
Also, the mixed child might carry on the foreign genes into your group's genpool, making up a longer term competitor for the genes of your relatives. To raise a mixed race children IN YOUR GROUP, the home- and heartland of your population-race-culture, means you foster a foreign and potentially competitive, even hostile element.
If you raise them in foreign territory, it might be an expansion, but in your own, you just threaten your relative's genetic and memetic success. The only argument would lie in totally superiour traits, which is rarely given, especially if talking about high standing European Europid populations and the choice for male semen females can have in the modern Western society.
After all, it is never just about your own offspring, but also other carriers of YOUR GENES and people being genetically related.
In the human case also phenotypically and memetically-culturally.
Both is not the same if you mix with a completely foreign racial person - especially not, if you are a woman, or a man who invests ONLY in that mixed children WITHOUT pure offspring.
Also, females are the carriers of the pure blood line. There is no other choice: Females get the pure offspring, or there is no pure future generation.
Males can mix, because they can be expansive and their 50 percent mixed ones are a plus, unless they spread the mixed genes in the home population, that can be a problem too.
But female who mix are always a greater loss, because they could very well carry on a pure bloodline.