When people move from an underdeveloped nation where their carbon footprint per person was 2.5MT/c to a developed country where they now contribute 8MT per person, that impacts emissions too. Now multiply by millions of migrants (either internal or international)
In fairness, you could ask the populace of the US, EU, JP, SK, etc. to cut their footprints by 50%, let's say, but neither they nor you realize what that would mean on the ground.
It's easier to ask someone who doesn't have something to give that up than the person who already has that thing. (Ask me to give up 10MM vs the person who has 10.2MM)