In FFXI they learned the errors of having an economy where durables like gear are allowed to propagate endlessly from user to user. It killed any reason to craft or gather the gear at all because the market for these items was super saturated and there was no solution to fix it. You could say "well crafting isn't important here" or "but we all run dungeons a ton anyway" but that's just the way the game is today at this exact moment. To allow us to trade these items endlessly would have a long term negative effect on the game. As it stands it was a wonderful strategy to make crafted gear turn into materia. It keeps much of that market flowing. The only thing we need now is the ability to convert gear from dungeons as well.