For those who want to level together, no one's stopping you. Just run the dungeons without the roulette. For the other roulette formats, you first have to unlock every item that belongs into the group. That ensures that all in the group, if eligible to the roulette at all, are eligible to the same content. I think it has to do with the matching. The group (pair) should be matched by the lower level one's limits giving the higher level a free pass to over easy content and "free" high exp bonus. No one forces anyone to do the roulettes anyway, want to run with friends, you're very much free to do so. You just have to pick the place to go.