Because they're less serious... A cyber attack isn't equivalent to bombing a city or invading with boots on the ground. And governments have done far worse than

Because they're less serious... A cyber attack isn't equivalent to bombing a city or invading with boots on the ground. And governments have done far worse than hacking some infrastructure without it sparking a war (eg: blowing up passenger jets, hijacking or destroying shipping vessels, assassinating officials - things where people die) War only happens when one government thinks it is the appropriate political tool for the job and they have the means to wage it.