Russia's been shot at for a year and still hasn't declared Ukraine to be an actual war. Ukrainian drones are literally flying into Moscow to assassinate Putin and he's still not declaring war. NATO trainers, advisors, spy planes, mercenaries, and special forces are operating in Ukraine right at this moment.
Countries will thread this needle every time when they want to avoid escalation. The US is betting that China won't be willing to escalate and so it can get away with having its East Asian vassals do the bulk of the work. It's betting that any conflict over Taiwan will be a limited conflict that is fought primarily over military targets. In such scenarios, South Korea and Japan can do a lot of damage without the US formally joining the war (because its defense treaties with Japan and South Korea only requires it to defend their land; it doesn't require it to defend their military assets sent against China).
I can easily see a future in which the US states, "we fully support our allies' committed defense of Taiwan and will do everything in our power to help them succeed. However, rest assured, American troops will not be fighting China."
East Asia is not Europe. Europe has a safe rear - the Atlantic and Mediterranean where Russia has no influence and where Russian conventional forces can't reach from the border even with no resistance. Europe also has fuel self sufficiency through North Sea and Norwegian offshore oil, and pipeline imports from the Middle East. They also have food self sufficiency. Most of all, any Russian Army advance will be through highly populated urban areas that will resist every step of the way and grind their field forces down.
East Asia outside China has no real safe rear. South Korean, Filipino, Japanese and especially Taiwanese supply routes
run through Chinese EEZ waters. The routes that don't, are under Chinese fire control. None of them have food or fuel self sufficiency.
The only way to supply Japan free of Chinese fire control - not South Korea or Taiwan - is a roundabout way around Australia unloading in Honshu, but that still leaves the critical forward operating bases of Okinawa, Kyushu and Shikoku isolated. Taiwan and SK are outright unable to be resupplied effectively.
It is also hard to enforce limited war in East Asia due to international airspace. In Ukraine, Russia and NATO are both limited by hard borders that define zones of conflict and non-conflict.
In East Asia, both sides can legally fly planes up to 12 miles from the borders in peacetime. There is no enforced separation of conflict and non-conflict zones. This means that surprise attacks are much more devastating and limited not by borders or legality, but capability.
