Probably because the average Chinese is far more skilled than your average illegal immigrant.
It's not just raw labor force. It's capable engineers, designers, architects, etc. How is illegal immigration going to solve this?
The amount of skilled people wanting to migrate to the US is almost infinite (unfortunately). If the US decided to open 20million green card spots, they would be filled.
If you bring in 20 million skilled workers, you'd have to bring in 200 million unskilled workers to support them.