There still is a reactionary part of Chinese society that is super obsessed with matters of class and status, or at least they are much more than Americans for sure.
Not having your child getting into a university can be seen as a social stigma, and parent of such child will be socially ostracised.
For such, having a child getting a place in US college for money with 100% certainty, is better than him/her taking risks with state exam back home.
The few standardized tests that do help in admissions like the SAT are just glorified IQ tests. Very hard to practice for beyond a certain point. Not that there isn't a whole industry of people cramming for SATs for nothing...
In most of mainland Europe you can easily pay peanuts to get into the absolute "top" schools. Yet few want to. American education is that much more valued.