Because that's the way it's always been for War? Additionally their cornerstone abilities are locked behind each stance, unlike PLD and mostly unlike DRK - Blood price/weapon notwithstanding. It doesn't cost War anything to switch stances - but it does cost them abilities. If you want to talk about the costs being imbalanced, that's fair, but it's an entirely different discussion. The main reasons Defiance/Deliverance are oGCD is because there is no innate damage mitigation from the stance and because War looses parts of their tanking kit when switching.
Grit is definitely the "in between" GCD/oGCD and should be looked at by most opinions. I personally have mixed feelings on PLD oaths, I suppose if forced to settle on something I'd lean towards taking it off the GCD but retaining MP cost as PLD (and DRK) both recover MP naturally via their DPS rotations. If you are needing to switch stances because a run is going south, the last thing you should be focusing on at that point is your DPS. Just make it easy enough to switch stances and recover, then switch back when stabilized.
"But War can negate damage penalty" Big whoopee. War doesn't go to defiance and pop unchained for a DPS increase, it's still a loss. Additionally, consider that you rarely tank in Defiance with Unchained for the full 20 sec duration. At most for 3-4 GCDs on the pull, and 10 sec mid-fight until we can switch back to deliverance. (for prog it's a bit more of an advantage but not by the time you have the fight "on farm"). DRK has good snap aggro with DAPS, so it's just PLD that's losing out. But let's be honest, it would still not be optimal for a PLD to pull even if there were no MP costs/GCD loss associated with stance change. It's still delaying their dot and they still have the weakest aggro combo.
For DRK, there have been a ton of changes mentioned in the tank forums, so I don't really understand why the OP would rather have "non-tank feedback and discussions" when the one's actually tanking & commenting have a pretty good handle on what needs to happen. Regardless, they need a bit of help defensively, offensively, and utility-ely. CD reduction on Shadow Wall, a fix to LD so it's not such trash compared to HG/Holmgang. TBN would be better as a 30ish sec CD with no MP cost to help alleviate the constant tug-of-war between mitigation & dps. And they need something to compete with Divine Veil/Shake it Off - be it either giving them back HW-style Reprisal mechanic or another copy/paste AoE shield. And finally I believe they should offer a bit higher DPS than PLD. PLD simply has too much utility and defense to warrant their damage output even by SE's own idea of "balance". I certainly don't want to see PLD suffer like they did in HW, but DRK is kind of in that spot right now and they need to do something. Otherwise it'd be better for all of us to just #deleteDRK and focus on having 2 really strong, desired tank jobs.