@Dominions Son
neither of which have anything in particular to do with the metric system.
Actually, they have everything to do with the "metric system" and this is the fundamental point that a lot of Americans fail to fully understand. The term "metric system" is linguistic shorthand for the International System of Units (formerly Système international d'unités) an international system that defines standards for everything needed for communication in a multilingual world.
Why does Europe use a 24-hour clock instead of 12 hours? Because that's the international standard under SI.
Why does the US not generally use the 24-hour clock?
Because SI, ie "the metric system" isn't in common usage in mainstream US society.
Why does the US military use the 24-hour clock?
Because it is more clear, a feature which is vitally important in the military, and the reason why SI has been adopted internationally in the first place.
This is the whole point. SI isn't just sizes of milk bottles and what temperature it is outside. SI covers the clock used, and the numeric system (because not all cultures do mathematics the same way), and standard marine communication phrases, and UniCode (a key component of digital text), and the functionality of the internet, and barcodes, and geotagging, and hazard symbols, and medical classifications, and so on and so forth.
SI is the international standard. That's what it means.
I'm not saying that no one in the US uses any of this, I'm just saying that it's not the default. And because it's not the default you can never be entirely sure, without checking, which system is being used. It's not enough to teach something in school; if people aren't using it every day they will be at least a little confused when they have to rely on it.
A perfect example of this is the 1999 Mars Climate Orbiter disaster when NASA lost a $327 million ($617m today) probe because Lockheed Martin was not using SI units, despite the fact that they had been standard in scientific pursuits in the US since 1975 and were specified in the contract. If more of those engineers were conversant in metric someone might have noticed that calculations of pound-force seconds and newton-seconds produce wildly different results.
There are also a number of lesser known incidents, both in the US and Canada, with some of the newer Canadian ones resulting from products coming from the US with American packaging that has not been properly converted* to metric. These include passenger aircraft receiving insufficient fuel or being overloaded, incorrect doses of medication (grains vs grams), and incorrect/ banned radio frequencies being used.
*[Note: Canadian law requires that imported products have metric bilingual packaging, although including US standard is allowed as an addition. If a company is operating in Canada, as most of the big ones do, they have to do this themselves before the product leaves the importing warehouse. There is no (legal) excuse for an American product reaching a Canadian user without proper labelling.]