Why is it stems are always measured by deg of rise rather than mm of rise?

mm of rise depends on the head tube angle doesn’t it?

I think because it depends on the head angle, a slacker head angle would mean a different rise in mm, if that makes sense, it does in my head

yes, they do come with a faceplate 😉

Ok I get that.

I still think mm would be easier to visualise than head angle and degrees of rise. I can’t do geometry in my head :/