Although this is a cooperative game, the average player treats combat as if it's single player. Instead of learning and understanding the fights as a whole, they learn it only as it relates to their specific job. If you ask someone what a particular mechanic does, they probably won't know it unless it directly relates to them. For that matter, players often don't know what their own abilities are for if they don't directly benefit their own gameplay (What's Shadewalker?) Most interpersonal conflicts between players during raiding happen when the fight didn't play out exactly the way that they expected it to in their head, forcing them to adapt and respond on the fly.
One of the reasons why dps roles are so popular is because you can get away with this type of mindset. There's just you and the target dummy, with the game throwing set mechanics that you've been trained to either respond to or ignore. You can treat healing like this as well, as the core of your role revolves around responding to a pattern of timed attacks. When someone takes avoidable damage or dies, however, you suddenly get yanked out of single player mode and are forced to respond. Healers don't get frustrated by having to heal expected damage. They get frustrated by having to heal unexpected damage. Granted, skilled healers have situational awareness and are good team players, but your garden-variety healer is capable of tunnelling on their whack-a-mole mini-game just as hard as any dps can.
Tanking is fundamentally different. Because the mobs are linked to your position, and everyone else's position is linked to the mobs, you are either directly or indirectly responsible for every event that happens on the battlefield. Your goal is to set up the fight so that it happens exactly the way the other seven players are expecting it to happen (irrespective of whether those expectations are incompatible with each other). In order to do this, you need to understand what every mechanic does, to have situational awareness of what every player is trying to do, to try and facilitate their gameplay, and to tie everything together as best as you can. Your job is to take seven independent single player games and wrangle it into one cooperative game.