US has 2x more troops forward deployed in East Europe for Russian contingency and decades worth of equipment stocked piled as result of cold war. There's also all of NATO. Versus TW where PRC concentration of forces overwhelm US air/naval assets and reinforcement is far away in time and logistics and there's no US allies in region willing to credibly contribute to TW defense.
The entire current TW defense strategy is to line up on the beach to be blown up based off 90s PLA invasion plans not 2020 PLA plans based off US stomping Iraq with high tech weapons and complete air supremacy. The "tough nut to crack" / porcupine strategy are asymmetric doctrine US think tanks want TW to adopt so US can respond it time. It's not reality, currently PRC can stomp TW before US help arrives. And it's questionable US has any supremacy within 1st island chain for TW defense. Also PLA isn't filled elites, certainly not combat roles, and what few there are will have strings pulled not to be deployed anyway. The entire one-child will disincentivize TW war meme is also nonsensical, PLA wants martyrs after decades of relative peace and frankly having a dead hero is going to come with a golden parachute for the family.
The real answer is PRC still prioritizes peaceful reunion because time is on their side.
Where as time is not on side of Russia, simultaneously because force balance could shift away for Russian favor and Putin has actually managed to credibly modernize Russian military that it's at peak power to pursue/restore it's security environment that has been devolving for the last 20 years. Ultimately, Russia is currently powerful enough to fight for her interests, because on balance capability is in RU favor. US isn't going to intervene, EU underinvested in defense while Russia invested enough despite how poor they are.