As nice as that would be -- believe me, I'd like it too -- I suspect the reason they can't is that we don't inherently have "this is the gear I am wearing for a job" data; we have gearsets, yes, but you can have no gearset for a job and just equip the weapon and job stone (and I know people who do this!) and then put on whatever gear you have. You can also have more than one gearset for a job; I have a Bozja healer gearset (to put on my gear with Bozjan bonus stats) and a Eureka RDM gearset (to put on Eureka-bonus gear), in addition to my normal gearsets.
What it would need to do is store your equipped gear at the moment you queued in (instead of just "job, at this level, with this ilevel"), and forcibly equip that gear when pulling you into the instance.
The two problems with this are how you handle things if someone has gotten better gear in the meantime and gotten rid of the old gear... and the fact that while a gearset doesn't take up a lot of memory, it does take up a non-zero amount. And this game seems have enough issues with hitting computing resource caps that I fear what would happen if the roulette system needed to take up, like, 4x more active memory than it currently does, just in order to store everyone's gearset data at the moment they queued in.
The alternative I see would be that it would just do the "equip recommended gear" for a given job to calculate your ilevel, and then when you queued in it would do the 'equip recommended gear' again.
This would have the benefit that gear-cheesing roulettes would no longer be a thing -- you can't switch down to a low ilevel to ensure you get Crystal Tower without stuffing all your level 90 Asphodelos stuff into a retainer or something! -- but it would also really mess with low-level roulettes; in the Realm Reborn level range, frequently it will (incorrectly) recommend equipping higher-level DPS gear onto a tank and so on, because of how things in the ARR level range are set up to be equipped just by "Disciple of War" or "Disciple of Magic" as opposed to Fending/Maiming/Striking/Aiming/Scouting/Casting/Healing.
But yeah, I agree; the implementation issues I see in it notwithstanding, if there was a good way to make it work it would be really nice. Especially for anyone queuing as one job and then heading out to, say, farm FATEs on another...
