Typographic units are non-metric and metric units are inconvenient in typography, because millimeters are too big for character sizes and too small for page units and centimeters are both too big and too small at the same time.
French (?) shoe sizes are numbered like 38, 39, 40, 41, etc. and the difference between these sizes are 2/3 cm, because it's a practical difference to mass-produce different shoe sizes. It's not metric, obviously.
Parsecs are not metric. Heck, light years are not metric; and look at how rich the unit is, it tells you a lot about the distance. Try to express the distance to Alpha Centauri in metric units and comprehend it.