Along with the other reasons, WoW isn't really a great example of a better queue system because the content all takes different ratios of players, which ends up being even worse because it ends up making certain roles more necessary than they were during leveling, and others have to struggle for spots. 20% of a group is tanks while leveling up, but in a 25man raid that has 2, maybe 3 tanks, that's 8-12% of the group as tanks, only allowing half of the tanks as when leveling up. Healers are 20% of a group while leveling up, but at max level there's usually an extra healer, 3 in 10 mans and 6 or maybe 7 in 25mans.

XIV at the very least manages to keep its party composition the same throughout content (except 24mans, which were never really high-end content anyway), which helps keep people from leveling classes and finding out there's not any room for them after they spend all their time leveling.

The ratio of tank/healer/dps isn't static, and adding another slot isn't just going to totally cure it. If dps queues get noticeably better, the people who switched to tank/healer for better queues will just end up going dps again and bloating the queues. If dps queues get worse, more people will jump to tank/healer. It's a balancing act, and rebalancing all content in the game just to try to alleviate a bit of dps queues seems like a terrible idea.