The reason they don't allow it is to stop people at level cap being in queue together to speed run for the bonuses and tomes. The idea behind it was to encourage more high level players to queue levelling content and help new players while providing a bonus incentive to do it. A lot of players are just greedy and have no interest in helping others do anything they just want their reward and to leave.

I think they should allow under 60 characters to queue together though so people can level up together.