Because demands are high and as soon satisfied no one needs it anymore, the problem is no item is relevant for long, crafting gear or materials, it is simply way to much, i keep saying it, no one needs each 2 levels a white gear, on top of every 2 levels a dungeon gear, while endgear you get with stones mostly and crafting is stronger in theory it is mostly used to skip up.

Main problem i see is how useless all becomes so fast, departing items is still not worth much since the materials you get out of are usless itself, i look at Guild Wars 2 here, rarely is anything to low in price, sure there are exceptions or useless items, but not to such degree, i still not know how they manage it but, every can be taken apart for materials you keep reuising or can sell more then at the vendors, or trown in the mystic forge, etc.

Items and gear simply has so many use and is worth something in any way, think this is the biggest difference here, on top of a ble ot sell under vendor price is one of the biggest issue in FFXIV.

I played alot of games but FFXIV has one of the worst market i have seen so far, undercutting in like 1 week, more other games do that to but not that fast, at least those i played.