How can there be so many chinese students in masters.

They are the ones who apply. Relatively few natives see the value in a taught masters program as most will either elect to get a job post-college, go to a professional school that allows direct entry or do a PhD instead. Those paths are seen as less risky vs dropping (or going into debt by) $100k on a short masters program that might not land them a job.

They have enough money to swing the insane cost of attendance. There are enough wealthy families (or at least middle/upper middle class families dedicated to saving for their children's education) from China/ndia that can afford the type fees these schools cost. And if not, plenty of banks are willing to lend to supposed future "STEM professionals".

Doing a tech/quant masters to get into technical or quantitative careers is probably the highest paying, least risky path for international students to enter the US workforce. STEM OPT gives them a min of 3 years of work visa and ~3-4 shots at the H1-B lottery at a higher success rate than those with just a US bachelors (there's a separate H1-B pool for US postgrads). Plus, tech companies and quant finance employers regularly sponsor unlike say IB employers.