It doesn’t help that the largest group content available, raids, doesn’t follow a 1:1:3 ratio of players. Even at scale. You can have a ten man raid or a twenty five man raid and you only ever need 2 tanks tops. Healing varies greatly with your raid’s gear level and the difficulty. I’ve seen 25 man raid teams healed by 2 healers or more but the average is less than 1 per 5 players.
Then you break up into five man content and now you have this abundance of DPS to tanks and healers. Tanks are the worst because you only need 2 and even in LFR it only queues 2 tanks for 25 people.
Even IF, and this is a huge IF, there was a consistent desire to play each role by the player-base, and even IF, every class was equally balanced so that you weren’t drawn to a particular class over another, we’d still have a shortage of tanks, then likely healers for the available pool of DPS who queue.
Now, people know you get faster ques with tanks and healers in your group. Most tanks and healers worth anything have a friends list a mile long. While they could queue by themselves, their usually queued with at least one other individual. Follow that formula to an extreme and you usually have a tank / healer queue together or a tank/dps or a healer/dps. So it grabs a tank and healer… it grabs a healer and a dps… those DPS shortcut the line because of the shortage of tanks or healers…and all the other dps get to fill that one individual slot that is left.
This plays significantly into the reason why when you have a group of 2 or more DPS it can take a lot longer to get a que. Your not eligible to get the tanks or healers that are grouped with other players that would make your group incompatible. If your 2 DPS, and you get 1 tank and a dps that are grouped… you can only get a healer solo qued…or vice versa.
Interestingly to combat this they changed the rewards they give the roles that are in high demand so that you can’t earn those if your grouped.
The other thing is this is why you get such disparaging variance in quality of tanks in queue because - anyone can play anything they want and the system has no concept of skill. You get a lot of players who DPS as their main spec who YOLO it as a Tank in queued content and have no idea what they are doing or the gear for it either they just wanted a faster que.
Its another reason you can sometimes find situations where a tank or healer leaves the group as soon as you enter the dungeon. Someone has two accoutns, their using their second account to skip the line for their DPS, and then they just drop the healer or tank on the other account. Because its a group in progress it gets priority for the next solo qued role and bam.
It all explains why queues are Fd up.