My puzzle is different: why is framing so expensive? The frames are just finished wood, which you could import or buy cheap at a lumber yard. The framing labor is often immigrants. Yet a good framing job can cost more than a refrigerator or washer/dryer set.
I'd say the answer just comes back to my original puzzle. The price is high because firms are not realizing economies of scale. Consumers therefore wind up paying for a ton of overhead (like the typical framing shop that is totally empty 2/3 of the time). The real mystery remains why market forces have yet to consolidate this industry.
Of course, by the time we figure out why this can't happen, Allposters.com may go ahead and do it.