eli5, Why isn’t there a nautical kilometer?
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Everyone *used* to have their own miles. There were Swedish miles, English miles, Italian miles… and there was a lot of difference between them. So if your charts told you to sail south for 200 miles, which miles were you using? Were you supposed to sail 200 that could be 420km (“old” English), 1234km (Welsh), 1507km (Prussian)… On land, you at least have landmarks to take a guess, but at sea? It’s important to know which mile you mean. It became easier to just use a specific mile when talking about the sea (the *nautical* mile) and set the to a value that happens to be useful at sea (one that makes calculating distances from a map easy).
Nowadays, *most* of the miles have fallen into disuse. Most countries no longer use their miles… Except for a small handful of countries which use very similar miles (a mile in the UK and a mile in the US are ever so slightly different), and specialised uses like ships and planes. Because one of these, the 1609-ish meter mile, is vastly more common than the other, we just call one a mile and we give the other a specialised name.
Why isn’t there a nautical kilometre? Because kilometre *has always been* unambiguous. A kilometre has always referred to a specified amount of distance. A mile, meanwhile, used to refer to all manner of distances.
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