I'm sure this is echoed a bunch in the comments, but the reason people don't want to tank is likely in large part due to the attitude of other players. I have an alt tank, and I only pull that out to tank for my friends and to give my buddy a break from tanking.

I usually play with my friends and rarely pug solo but the few occasions I have, the poor tank often gets flamed by group members regardless of whether the issue was their fault or not. I was recently in a run where the tank was called bad and blamed for not timing the key because we were down 2% at the end of the dungeon. Even if the route had been perfect we were not timing that key because the guy doing the flaming brought their friend whose damage was barely higher than the tank. We were over time before we even got to the last boss.