Visas are a pain in the butt. This year in April they had 233,000 applications for 85,000 H1B visas available. So that's a lottery with a chance of 1 in 3. And even then, your developer can only start in October after successfully filing in April.
China is a huge time difference, and language is always a problem. Plus, Bay Area startups want to move fast so need good communication. That's why everybody is still hiring local talent.