Why do ship's captains and airline pilots still use 'nautical' miles and 'knots' to indicate distance and velocity, instead of more modern, mostly universal, metric measurements?

Why do sailors and air navigators use nautical miles? Aren't ordinary miles good enough?

THE ORDINARY, or statute mile is an arbitrary length of no particular significance in navigational calculations. However if I move one nautical mile along a meridian of longitude, the 'north south' lines on a map, my longitude changes by one minute of arc, which is one sixtieth of a degree. If I move along a great circle, the shortest distance between two points on the earth, one nautical mile again is one sixtieth of a degree. Over long distances this gives a link between distance and latitude and longitude. Over the short distances of weekend sailing or private pilot flying the link is rarely of any significance. One nautical mile is 1.15 statute miles, and a speed of one knot is one nautical mile per hour, or one degree along a meridian per hour ...
The big point about using nautical miles (and their corresponding speed unit, knots) is to make chart reading quicker. Charts use Latitude and Longitude, because, well, that's how you find things. Therefore they have the Latitude and Longitude grid printed on them. The grid spacing that equals one minute of latitude also equals one nautical mile. This means you can instantly judge multiples of miles off the chart with any handing measuring stick (such as a thumb). This is invaluable on a heaving chart table to a (probably heaving) navigator.