I won't appeal to geo-politics or moral ethics like some other responses, but simply the historical tradition of warfare.

Both in Europe and Asia, there has traditionally been a distinction between combatants and civilians. Wars were often fought within cultural limits, determined by era and society, of acceptable conduct.

Total war and war of annihilation is rare. More often, there is a clear military objective, like the capture of land or the end to a certain enemy policy.

When facing a military threat, it is important to show resolve and competence, but wanton destruction is rarely beneficial. Responding proportionate to the threat is a way to contain an enemy without escalating the situation.

If the enemy tries to capture a city, defending it and destroying their attacking force sends a message about your nation. A campaign of retribution against surrendered or retreating forces just makes you look petty. And counterattacking against civilians makes your side hated.