Because China has been an advanced civilization - in the sense of a settled kingdom able to support large populations on marginal soil - for a very long time. Everyone's heard about the Great Wall. IMO, China's first major practical engineering feat was its building of the 120-mile Han Canal 2500 years ago, which served simultaneously as flood control structure, waterway and irrigation aid.
The US population has grown far faster than China's. In 1800, China's population was 300m compared to 5m stateside. Today, China's population is about 1.3b vs the 320m stateside. The Chinese population is about 4x what it used to be, whereas the US population is about 60x.
By comparison, around the time Christ was born, the Roman Empire included ~5m people vs the Han Empire's ~60m. In other words, it has taken China 2000 years to increase its population 20x, whereas it has only taken the US 200 years to expand its population 60+x.
China doesn’t have an ‘immigration’ problem like we do............B^)