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Because "ton" was an established amount in trade and shipping (though with significant local variations), that was later adjusted to fit into the metric system and standardized. Hence why people specify "metric ton". There was simply no need for people to change their terminology when they already had good monosyllable.
Similar to "mile" which in metric countries were brought into the standard and defined to be 10,000 meters. While these days kilometers are almost always used for long distance in all official uses, people's habits are still to talk about "miles" when describing how far something is to travel. E.g. "I live roughly 2 miles from town" flows better than "I live 22 kilometers from town".
Edit: Recent example use of metric mile: https://www.nrk.no/tromsogfinnmark/kirkenes-if-og-norild-il-ma-reise-100-mil-for-a-spill-hjemmekamp-i-fotball-nm-1.16338078
Metric citizin here, nobody uses "miles" here. We just go with the metric system:
1000 metres = 1 kilometre
We use the wording from your example "i live 22 kilometres from town"
That's fine and I believe that is true for your country or region. Though remember there are other metric countries. E.g. the nordics still stick to it.
Interesting trivia piece is that in swimming and running the 1500m is sometimes refered to as a "metric mile" because a mile is 1600ish.
You're half right. There is no "metric mile" at least not officially.
But the reason ton/tonne worked in both is that a metric ton is 1000kg and an imperial tonne works out at 1016kg which is close enough for damn near everyone who weighs shit by the ton/tonne.
But then the Americans and Canadians had to create a stupid hybrid and define the "ton" as 2000lbs (About 907kg)
Thanks for the extra information. But we do have metric "miles" in metric countries. Norwegian spelling, for example, is "mil". Icelandic is "mìla" Etymology is from the imperial mile, again from latin.
Example from official national dicitonary: https://snl.no/mil