Inertia and things that are really, really inconvenient otherwise. Here in Saskatchewan, the "grid" roads serving rural Saskatchewan are actually laid out in a 1 mile by 1 mile grid, enclosing 1 section of land (640 acres). Even equipment without odometers can follow directions like "4 miles north and 3 miles west" by simply counting intersections.

By distance, Saskatchewan has approximately 1/3 of all the roads in Canada despite having only about 1/35 the population. Miles are not going anywhere, even if everyone gives total distance travelled and highway distances in kilometres (or approximate travel times).