Because we're not having a moral argument but rather a practical one. The fact is that China emits much more CO2 than other countries, even after you account for exports (ie, CO2 emissions "offshored" from Europe and North America). Moreover: while emissions in the west are falling, the are rising in China and India and the developing world, as they must, because they track rising standards of living.

I don't understand the "25 smaller Chinas" argument. China sets statewide emissions policies. That's a reason we pay attention to country borders here, rather than considering an undifferentiated mass of humanity as a map scatterplot.

Ultimately, the reason to point out the gap between China and Europe or North America is that if you concentrate climate mitigation strategies on the west, you won't actually do much to solve the crisis. Fair's got nothing to do with it.