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Because Nautical and Geographic distances were based on astronomical measurements (made with a sextant) since there was no easy way to measure a distance between to points, or a number of paces, etc...

IIRC, knots is a term for speed first used in "the olden days" of sailing.
Speed was measured by dropping a log (floating object) tied to a cord over the stern. The cord had knots tied in it at...