Because evolution in irrelevant in our modern world. It is not survival of the fittest. Everyone survives regardless of how fit they are. Modern medicine and social welfare networks ensure it. Even the most deformed broken and genetically diseased people can now live normal lifespans.
Additionally, all the places that white people live have embraced feminism and cuckism which means all their women are too busy working and going to school to have babies. Thus even when whites pair with whites they are not reproducing sufficiently to replace themselves. White women have mostly "gone their own way" and chosen their own selfish desires to ride the Cock Carousel and "get good jobs" over reproducing to propagate their race.
Meanwhile, in areas where reproduction rates are highest (eg. India, China, Africa, Middle East) there are insufficient white men to MOG the local population of men, and thus the men there still stand a chance. There is also little to no feminism so women still consider their primary role in society as baby incubators and they will bear the future generation with pride.
Evolution has failed. Advancements in society - social, medical, cultural - are currently and will continue to lead to the degradation of the race as the lowest IQ and ugliest races continue to outbreed the tallest, most historically successful, and best looking one overall.
It's a goddamn joke.