I'm not so sure.
Why haven't they gone after Taiwan already?
Yes, they're preparing for it but what are they waiting for?
I guess is that they don't really want a world war and see a risk that the U.S. will step up to defend. We don't know that the U.S. will, but there is that famous ambiguity that they might.
In Ukraine on the other hand it was stated clearly, neither the U.S. or Nato would directly fight Russia.
Perhaps it was a mistake that the U.S. didn't leave direct conflict with Russia on the table. If anything might have stopped Putin, maybe that would have.