Might you have heard the old saw "Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands"?

Military spending is similar - you want security at the minimum cost.

Unfortunately, military conflicts (real and potential) have vastly more complexity and uncertainty than the gravity, wind, traffic loads, etc. that a bridge has to withstand. And even a nation's best experts (generally its senior military officers) generally have motives at odds with minimum-cost security for their nation.