Now, there probably is no such thing as an elf, and if there is, it is probably either benign or miscevious, so the potential reward of stopping an eldrich attack is low.
But we've decided that cost/benefit analysis doesn't apply to Defense, so we better get on it right away. Need I remind you that we are utterly unprepared to defend ourselves from attack by virtualy any mythical creature?
And whatever we decide to use against elves probably would come in handy if we were attacked by gremlins.
Seriously - there is a certain amount of money available for defense. We aren't comparing cost/benefit between free market investments and defense, we are comparing cost/benefit between one form of defense and another. Spending it on technology that might be effective against a threat that is unlikely to materialize may not be as prudent as spending it on known effective defenses against known threats in progress.