|A few reasons from a scientist point of view:
1) Regardless whether or not the US builds missile defense shield, China continues to build more missiles that carry neclear war head(s). China possesses more than half of the world's known rare earth mineral deposit containing both Lanthanide and Actinide Series (Pu and U in the latter). Mobile land and sub-based missiles will likely survive the first strike. It is unclear how many they have built (its first one was almost 40 years ago, then H-bomb, and neutron bombs) how many more they will build. China's economy if based on PPP (purchasing power parity) estimate is already many times bigger than ex-USSR. So it is safe to presume that it can at least easily make as many as ex-USSR in the coming years.
2) 10,000 nuclear explosions in China are NOT sufficient to vitrify the land owning to geological and geographical complications, eg, China has far more mountains than US. Theoretical calculation suggests that such vitrification process requires roughly 192,0000 explosions. However, the nuclear fall-out (ie, airborne radioactive particles) will drift acoss pacific ocean and deposit in the US mainland, destroying every live-stock on the way. Everyone knows why a balloon can drift from China to the US, not the other way around.
3) The most compelling reason, perhaps, is the fact that after the first strike, the remaining Chinese missiles will be launched towards all major US cities and industrial centers. Despite the missile defense shield, a certain % will pass, land, and explode. The impact of 9/11 was shocking to most in the world (eg, most Americans probably delayed their retirement for a few years!); one cannot imagine the impact of nuclear explosions. Western civilization values human life and prosperity far more than Eastern civilization, such an nuclear exchange will certainly result in the destruction of both civilizations or the return of the dark age for many years to come; nevertheless, the rich or haves have much more to lose than the poor or have-nots. The US or American people is highly unlikely to take such risk of a certain known outcome. Good life is just too important.