You say there is no reason, but there is. Too many people play DPS as opposed to Tank/Healer for a number of reasons. Here is a list of reasons why DPS queues are long for dungeons:
1. Majority of the playerbase is doing endgame content.
2. Tanks and Healers don't go to dungeons exclusively to grind, they also go to Frontlines and Palace of the Dead too.
3. Cracked Crystals incentive lost its effect over time once people got enough Materia.
4. 60% of the jobs in this game are DPS. This means the odds of enough people choosing to play as DPS over Tank and Healer to produce an overflow of DPSers is high.
5. Sometimes people feel more pressure from their fellow players as Tank or Healer. This can discourage them from playing those roles.
I could go on, but what do you honestly expect the devs to do about any of this? They can't spend every waking minute constantly finding and adding incentives to get Tanks and Healers to queue for the dungeons more often. The problem is too many people play DPS, and honestly that is a fact that will probably never change. I mean you can't expect the devs to discourage playing DPS jobs in order to get more people to play Tank and Healer now can you? Besides, it's a common thing for the larger portion of the playerbase to DPS. That's just how online gaming is.