One of the problems here is that, at least for levels 1-49, it's virtually impossible to get gear (at least full sets of it) for the class you're playing while it's still useful to you, stat-wise. On my ACN/SCH character, for instance, I ran the first trio of low level dungeons every day for nearly a month before finally getting the Acolyte's Robe. Of course, by that time, all those low-level dungeon runs had raised me to something like level 42 or 43. I wasn't going to pass on the item I'd spent a month trying to get (and which was still stat-wise useful for my level 15 CNJ and THM classes) just because all my attempts for it had already raised my scholar to well above the item's level.

I guess it'd be a bit different at endgame, where you're gearing up rather than leveling up. There you'd at least have the option of focusing on a particular item or set and not moving on until you complete it, which is something that's not even possible while leveling. But even then, most people want more variety in their game than that, and want the ability to work on several different things. That's going to turn into looting the ones that take longer due to the vagaries of RNG after the ones that come available sooner, even if that's not the order of their iLevel.