Actually, the Duty servers used to pool from all servers. The problem from that was lag. Players from Japan, Europe and the Americas routinely played together inciting lag the likes of which hasn't been seen since. They instituted data centers to reduce the lag. There's leftovers of this old integration in the Duty with the language selection. Reintegrating the Duty servers would hasten queues but also bring back the old lag. Good luck dodging AoEs.