Because the real world is not a Civ game.

Believe it or not, China isn't Russia. I can't believe I'm in the position of defending the PRC, but the PRC doesn't want the international order destroyed by reckless and unrestrained warfare. They just want to replace the West as top dog in that order. They'll bully and bluster, just as the US does when the carrot doesn't work, but, Taiwan aside, they don't have any desire to start an expensive and pointless war.

Because first of all they would then have to care for all of Russia's very nationalistic citizens. Second, why would they do anything while they can just sit and watch Russia piss away their entire economy and military?

Russia is an important geopolitical ally for China. Tension between them is not advantageous at this time.

Also, Americans love dunking on the Russian army, and while it maybe wasn’t as formidable as we thought, it’s still significant. It’s not as if it would be totally free.

what’s preventing China from just taking ALL of Russia

What for? Russia is already drifting into becoming a China's satellite state. Besides, there's another resource-rich, sparsely populated, 99.9% Asian country right by their border, with barely any security and which would've been part of China already if not for some weeb. If they are going for conquest, Mongolia would be the second target right after Taiwan, but attacking it would tip off Russia to go all in on defense.

Russia would never threaten China with nukes, because 1) China ALSO has nukes, and 2) China has been the only thing keeping Russia afloat recently.

The problem here is the amount of them and population density. Just one bomb dropped randomly somewhere in China would probably cause more casualties than the entire Chinese nuclear arsenal targeting the most populous Russian cities. And Russia has an order of magnitude more...

You're right, nobody wants to defend it. There's nothing there worth defending. I mean, there's Vladivostok, but it's not really worth going to war over. They could take a sliver of land at the Russia/NK border so that they could build a port, but I'm sure they have no issue with river traffic as it is, or just trucking into North Korea to use one of their ports. I'm sure China funded their construction anyway.

Russia would never threaten China with nukes, because 1) China ALSO has nukes, and 2) China has been the only thing keeping Russia afloat recently.

But it would have to be a scorched earth kind of invasion. The kind that pisses off basically everyone, because it leaves every single Russian, military, or citizen, dead. They'd have to come in, take everything, and kill everything. Take the land.

First of all, if you're being invaded by an army planning to genocide your entire population, then you have no reason not to use every weapon in your arsenal. If the options are A: China kills 100% of your populace or B: Launch nukes and even 1% of your populace survives whatever follows, then B is the most rational choice.

Secondly, there's no reason to assume that states will make rational decisions to begin with. I'd say the current state of affairs in Ukraine is a very good example of that in action. So even if China wasn't planning to genocide all of Russia, even if it was some kind of "benevolent" invasion where they were going to tiptoe around the flower beds, gently pry Putin out of the Kremlin, and basically leave everything the same except that now Russians pay for groceries with renminbi instead of rubles... there's still every reason to imagine that Putin and his top brass would still launch nukes on the mere principle of the thing.

So no, let's not glibly plan for a fast forward on nuclear Armageddon, thank you very much.

The only thing of value in Russian territory is mineral resources, not the territory itself. China has vast, unpopulated territories (check a population density map). If they deem the minerals not worth the conflict, why bother? They can just buy whatever shit they want.

On a less deranged take, there's definitely potential to mend the Sino-Soviet split. Their interests and capabilities dovetail quite a bit, but I suspect unification is wildly impractical for any number of cultural and historic reasons. OTOH, if they presented a Warsaw Pact-style alliance, perhaps using the cudgel of mutually assured economic destruction instead of nuclear destruction, that's a hell of an act for the West to try to follow.

The US would get involved, two advisaries attacking each other would give the US opportunities to leverage influence and destabilize. The dangerous thing is that they have nukes so there is a delicate balance when trying to destabilize while ensuring advance weaponry does not fall into the wrong hands.

I would have much more respect for China if they used that as the bargaining chip to force Puttie to cut the shit and end this thing

Russia has kinda shitty land afaik. Not a ton of resources either, iirc. I believe a decent chunk of it is tagia forests and (soon to be) swampy permafrost zone. Not that much of china's land is great for stuff like farming either. Bragging rights, I guess, but other than the people, I think most land is of little use.