Because a tank requires 7 vuln stacks to take the same amount of damage as another player. However tanks also have almost twice the HP of other players, to take percentage health bar equitable damage a tank has to have 13 vuln stacks (remember it caps at 8 in the modern game). Then on top of that tanks are overflowing with defensives, putting up a single 20% defensive (so rampart, your 2 minutes or your short mitigation) means you need 19 vuln stacks to take HP equitable damage compared to a caster AND tanks can fully heal themselves near twice a minute
You aren’t talented and you don’t have a right to hold the rest of the party hostage, the game simply doesn’t permit you to die because of your ridiculous defence
Throw yourself off the map next time you take more than one hit every minute and we will see how much more “talented” tanks are