This is similar to my own opinion that items, weapons, and armor need a technology level, which helps explain why some are just better/worse than others. Crappy weapons like clubs and bottom tier armors could be found in the most primitive cultures, but almost nobody would use them outside of that unless they had no choice. On the other end of the spectrum, firearms, rapiers, and top tier armors should only be found in the most technologically advanced cultures, explaining why they're better than most other options. This allows the DM to easily limit what's available for purchase to PCs.