the simple fact is that there are more students from China to US over the years because more students can afford the tuitions, while the H1B visa quota remain the same; also there are more and more students from global as well to compete on the H1B quota. so most of the students have to go back simply because there is not enough visa for them to stay.
https://www.uscis.gov/news/news-releases/uscis-reaches-fy-20... 2018 H1b visa total quota is 85K in total for all students from all countries.
while usually <20% of H1B visa goes to people from China. https://www.vox.com/2017/4/13/15281170/china-india-tech-h1b-... so, each year, it's probably around 10K to 15K students from China can get H1B visa, compared to a total of 300K students in China, that basically means about 90% students will have to go back.
it's more about fast growing number of students from China, also working in US requires H1B visa.