TimR formulated the question :
It has never made sense to me that the probability of something conductive
landing on the blades was high enough to be detectable. Of course it's not
that hard to do it however the inspector wants.

One problem with that is reading a KilloWatt meter.

European outlets don't have that problem. The pins are insulated halfway
such that the tips don't make contact until they're completely inside.
Ground is top AND bottom IIRC, and pins side by side.

I think the real issue, not yet mentioned, is that a child can plug in a cord
with his/her fingers on the blade. That is FAR more likely than dropping a
paperclip on it.

Oh yeah, that one too, but not just for children. The idea that the
thumb on top might contact the hot blade when plugging or unplugging.

Many possible reasons, but none compelling enough for NEC to mandate.