It's for manufacturability, not design... Airstreams are now a premium brand, and it's quite expensive to develop and maintain a mirror polish. Someone buying a $150k polished trailer expects it to be completely unblemished.
You can still polish it yourself, it only takes about 80 hours of manual labor...
Also any road damage or wear will be much more visible on a polished surface, which is not a premium look.
I thought earlier-era Airstreams were buffed and clearcoated before initial sale. My only evidence is vintage photos, and the fact that some owners "restore" the mirror finish.
Well you will see big rigs with the polished tanks on the road already. They are typically cylindrical and even the back end is bubbled out so you do get some shine it’s not like a direct beam of light hitting your eyes.
You can still polish it yourself, it only takes about 80 hours of manual labor...