It’s the same insanity gun owners use justifying bringing weapons everywhere they go.
I simply don’t buy the argument, though I do see your point about deterrence
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.