Why do tanks exist?

[FallenWings]

Is there a reason why tanks shouldn't be staying in their tanking stances?

To deal more rDPS at an insignificant cost to survivability.

Although there's no excuse for tanks wearing STR accessories at the moment. But tanks not using tank stances are not all using STR accessories, your example isn't the norm but an outlier of baddies being bad.

Sometimes focusing on yourself gives more to the raid than obvious 'party supporting' utilities.