One, you are conflating IQ and g, which are not the same thing. IQ is a proxy for the measurement of g, but it does not measure g directly.

Two, the military does not administer protected IQ exams, but rather, the ASVAB which correlates with IQ.

Three, our entire global society does not revolve around who can do what for the military.

Four, what can someone with an IQ of 84 do that one with an IQ of 83 cannot?

Five, the US military has plenty of uses for low scores. If one can't peel potatoes, then I'm sure the military would just send them out to stomp for land mines. However, Federal law is what disallows this, not the military: https://codes.findlaw.com/us/title-10-armed-forces/10-usc-se...