Because nobody wants to declare war on China. It’s really that simple.
There isn’t really a rulebook for what countries can and can’t consider an act of war. (I mean there sort of is but it’s not enforceable because who can enforce it?) A country wants to, it could use a minor diplomatic incident as an excuse for war, or it could choose to ignore a major border incursion. For example, Russia could easily claim that the U.S. has committed an act of war by arming and funding Ukraine, but that would effectively commit Russia to actually going to war with the United States, and despite its bluster, Russia does *not* want to do that.
If a country doesn’t consider something an act of war, it’s because they did the math and decided it’s not worth it. Going to war against the world’s second largest economy and third largest nuclear power is going to be a bad idea unless it’s unavoidable. In other words, the people with the power to decide decided that it’s not worth it to potentially start a nuclear war with China over cyber attacks.
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