It's a bad idea, and not done in SMF for a few reasons:
1. Context. Maybe it's handled that way in English, but in other languages that might make translation difficult.
2. Efficiency. You should never load a language file you don't need - and ./ won't work in 99% of all cases either.
3. Phraseology. It is less than optimal to have every error message phrased the same way - it gets old. Using slightly different phrasings for each message (when they are never displayed in a group/together...) makes them "feel" a lot better.
4. Dependencies/Upgrading. What if, later, a string you are using in one language file gets removed... or renamed... or changed? What if a mod or theme uses it differently?