Because that is way harder than it sounds. When you hit lvl 50 in a crafting class, you can't automatically craft high level items. Item level 55 gear counts as one-star recipes, and can be tricky to make HQ, even if you gear up in full level 50 crafting gar, and they tend to require the harder to acquire gathering materials. Item level 70 crafted gear counts as two-star recipes, and those are strictly impossible to craft until you load up on crafting materia and probably some good crafting food.
Item level 70 armor is also the point where you need to start using coke, potash, etc in your crafting recipes. Originally, you needed to buy these with philo tomes, at 150 tomes per item. It took nine each to make the materials to make a piece of armor. So it took 1350 philo tomes to make a single piece of ilvl 70 gear, be it a piece of main body armor or a pair of gloves. It was significantly more expensive than Darklight gear. And you had to make that gamble that it would turn out HQ just to have the same base stats as Darklight.
Nowadays, you can buy one of those items for 2500 GC seals, or hope to get one as a treasure drop in the extreme mode dungeons. And you still need nine each to make an ilvl 70 item. It can be cheaper than before (in some cases, dirt cheap, thanks to a lack of demand for minium or a few others), but it still takes some money and a lot of crafting materia to make two star items.
Making three star items, ilvl 90 gear, is insane. The requirements to just get the recipes, as well as the special equipment needed to reach the minimum crafting requirements, is significantly harder than making HQ ilvl 70 gear. Even if you obtain those requirements, ilvl 90 gear requires special components that can only be bought with myth tomes, and other components that require two-star gathering materials, which are their own nightmare to acquire. There is a good reason that ilvl 90 gear is currently really, really expensive. There are probably only a handful of crafters on each server who can make it right now.
Honestly, it is far faster and cheaper to gear up normally through myth tomes and loot from CT and the Coil. Crafting gear is for cosmetic purposes or for people with the gil to buy it and meld a lot of materia onto it to make it better than standard ilvl 90 gear. In which case it is probably superior to ilvl 100 gear, but not as good as ilvl 110 gear.