OK, I complained to a Norwegian friend about the strange metric sizes that were made to match awkward export sizes, and I learned some interesting things.
2x4s used to be 2 inches x 4 inches, but when drywall became popular the wood was reduced 1/2 inch in each direction to fit with 1/2 inch drywall. The name 2x4 was already a standard name, and as I said earlier it sounds nice to say it, so the name stuck.
Here's the funny bit with Norway - what do they call a 38 x 89? They call it a "to ganger fire" - a 2x4! The same name as in English, even though inches are a very uncommom unit here. (Although TVs and bicycles are measured in inches, or "thumbs" as the Norwegians say (tomler).
So, it seems even though 2x4 is wrong, it's a nice name and it's international, even in a metric country. Which makes the name twice as wrong in Norway as in the U.S.
I can live with that.