Roulettes are not for you. Roulettes are for new players who want to get through old content. The reward is a bribe that they give you so you'll participate in Crystal Tower for the 584th time so little Sprout Joe can get through the MSQ.

If you could just opt out, the new player experience would immediately go to shit, as no one would want to do the less polished early MSQ content. Queues would take forever, and people would give up on the game because they cannot progress through the MSQ in a timely manner.

If you want roulette rewards, understand that they are not for you. They are a tradeoff that you're getting for making the queue move for people who are not at max level. If you want to run specific instances, queue for specific instances. You cannot have your cake and eat it too.