OP perhaps you should go check out the various testing people have done on parry.

A tank with 0 parry equipment only parries like 4% less than someone stacking it (it's like 7-8% without parry gear, 11% with full Parry). Meanwhile a tank that stacks Crit/det does way more damage than one with just parry gear.

As others have pointed out. You can't parry everything, and a lot of tank busters or raid wide damage is magical (Hyper compressed Plasma, Discoid, Holiest of Holy, etc). Go ahead and test it if you want. Take a WAR and have him use Raw Intuition on a bunch of moves. Only attacks treated as physical can be parried.

And in this game, the best mitigation is to kill whatever's hitting you faster. 900 parry won't save you. Pushing 100-300 more DPS to skip phases or meet DPS checks will