It's not. Cultural similarity is though, that is: almost everybody has to pull at least their own weight. If you have identifiable groups who do not, you're laying the ground work for severe conflict, because those who provide will at some point ask those who take to also provide - and they won't like it when the answer is "why should we?". A successful system doesn't require them all to have identical cultures, but the input/output has to be similar.

Ethnicity just happens to be a good predictor for culture, and culture happens to be a good predictor for the ability to be a net-contributor in a modern society. Ethnic homogeneity predicts cultural homogeneity.