A tank’s job is to protect the group. A big part of that is controlling the enemy. A big part of controlling the enemy is staying alive. Tanks have a lot of tools to stay alive, and mastering those is a major component of learning to play a tank. On the other hand, some of these tools are on long cooldowns, and on some encounters they are intended for use at specific moments in the fight. Furthermore, staying alive isn’t the sole responsibility of the tank, because there will always be one or more healers present whose job it is to keep the tank alive. As such, staying alive can’t be the only thing tanks have to focus on.