Killing people is also a good way to create more enemies, which a.) you then have to dispatch and b.) have a chance of killing you. There are very large second-order efficiencies to creating weapons that only kill those who pose an immediate threat to you and minimize collateral damage. Remember that the military operates as an extension of larger foreign policy goals, and it's far more efficient to never create an enemy in the first place than it is to get rid of them.

(Sometimes I wish world leaders would realize this, but a lot of foreign policy isn't actually about advancing a nation's strategic goals, it's about providing an outlet for aggressive tribal emotions that are usually suppressed within the population.)