"every single Chinese parent would rather their kid go the oxbridge route than any chinese school" - citation needed!
The oxbridge (as in "the bridge of the ox", a very logical outcome from "the ford of the ox" and "the bridge of the cam") are more of commercial enterprises than teaching institutions, and the preference for them are first and foremost the result of commercial advertising. The business model in here is to attract a great mass of pupils, be very selective about it (in order to have high-profile prospects), be very expensive (because the networking happening there must worth the hassle), then selling the achievements of these high-potenial strictly-preselected material to others as the results of its own merit. The research, the eccentric curriculum, or any other quirk that sets these institutions apart from others are just secondary. That is the naked truth.