[nradov]

The geography around most of the China/Russia border sucks. It would be tough to push through a mechanized invasion force even with numerical superiority. They previously fought a few limited border skirmishes with neither side able to make much progress. Even if China somehow managed to take part of Siberia the technical challenges and logistics would limit what they can actually extract. And of course the Russians would nuke Beijing before they surrender part of their core sovereign territory.