("Why is a nautical mile 1 MOA and a statute mile something else" is the more interesting question, IMO)
Lets handwave a conspiracy theory:
The Sumerian Ku was 518.5mm. A SAR-kus (60*60 kus) 1866meter is nearly same as a sea mile by 0.7%. One ku per second equals one sea mile per hour, equals one knot. So ancients did already know the size of earth by one and a half digits in 60er system (1/120). This required a mathemagical wisdom, like knowing how to compute square roots and pi to several 60system digits behind the comma, that was lost after 333bc.
Weights and measurements got mostly smaller and seldom bigger over time. The Sumerian mine lost 10% to become the lbs. Kings did declare the weights and length measurements at their time. Make the rod a bit shorter, if you have to hand out parcels after an invasion, and you have a good loot left for yourself. Use the bigger weights, after asking for ransom of a city, and even throw your sword into the balance, with the words VAE VICTIS! But making the weights bigger was a seldom exception. More common was the gain in declaring them smaller.
You're correct that the nautical mile is one minute of arc across a great circle and as such is a meaningful unit to a navigator.
Omitting that seems ludicrous. Without that, you're really answering the question, "why are they called 'knots'". Which is interesting, but completely different from the question of why you use knots.