Because liberalism is a mental disorder. It is immune to logical argument.
Liberals believe their mantra "diversity is strength" to be axiomatic - so self-evidently true that it cannot be debated, even if it flies into the face of reality at every moment. You can point out to them that in social circles and the workplace, like-minded people with similar outlooks and cultures tend to get along better than people whose attitudes and worldviews are completely alien to one another, and therefore the same thing applies to societies and nations, but it doesn't matter. When something is dogma, facts don't matter.