> Back in the day my country still had obligatory military service for the oldest male child of the family. Everyone had to do this. IQ tests were used to sort people coming in.
With due respect for your country, which country you're talking about is highly relevant. There are <5 countries with militaries comparable in efficiency to the US military and only Israel is really soldier-per-soldier competitive. That's not a criticism of your country--your country likely spends its efforts on better things--but if we're talking about what's effective for a military, US military practices really are a reasonable gold standard.
That starts with the fact that the US doesn't do compulsory military service, because soldiers who don't want to be there aren't effective soldiers (note that Israel does do compulsory military service, which is because they have different problems than we do).
> The military does not only need highly functioning autonomous operators. There were many more menial positions in cleaning, catering, hauling and warm body canon fodder than there were scouts or special forces.
Thinking that "warm body cannon fodder" is something the military needs demonstrates a bit of a misconception on how modern militaries work.
Cleaning, catering, hauling, etc. need to be handled by high-functioning people as well, because in a warzone, attacks on things like cleanliness to reduce morale, or attacks on food and other supplies, are effective attacks.
All this is to say, that if the US military doesn't want low-IQ folks, there are good reasons for that.