It's mostly economics and a bit of societal incentives.

For an American CS graduate, the opportunity cost of joining grad school is too high. It makes more sense to join the industry immediately and start paying back the massive loans.

For an Indian college graduate, it makes more sense to spend 2-5 years in the grad school if it means getting a job in the US later. With little or no loans to pay back, the earning potential is worth the extra few years of study.

Also on average, Indians and Chinese people tend to value higher education more than their US counterparts. Anecdotally, I have met few Americans who would prefer their spouse and children to go to grad school instead of working full-time after college.