Because rich countries brings their products from somewhere else, they don't have to produce them, they don't have to deal with the environmental impact of producing them.
This is true. China's environmental impact is 22% lower if you exclude their exports, and many countries in Europe would have 30% higher impact if you include their imports.
That's a common misconception, China is not even amongst the top 50 countries when it comes to industrial production value per capita. And most rich countries are near the top:
