@DekaBlack @LilahTovMoon because there isn't a compelling reason to switch all at once. Add in a strong cultural bias towards contrarian behavior, and you've got a recipe for the switch taking a long time.

I'm more comfortable with US standard and customary units for human centric things, and tend to use whatever system is more convenient for everything else. In the machine shop I use it's primarily inches, because the mill and lathe have imperial lead screws.