In an optimization problem, there is one stat that you try to maximize. The other stats are generally constraints that you have to meet, but not maximize. Crit and det are examples of things that you try to maximize (depending on whichever one gives you the best dps return). Accuracy (and to a lesser extent skill speed) are examples of constraints, with cut-offs that you are required to meet in order to avoid losing out significantly on dps.

The problem with Parry is that it isn't something that you want to maximize (i.e. it doesn't increase your dps), nor does it act as a constraint (there is no minimum parry threshold required to survive an encounter). Average mitigation tends to be more important in attrition fights. But this game isn't designed like FF1, where you need to get through Gulg Volcano on only 4 casts of your WHM's Cure 1. As a result, we tend to just ignore parry and minimize it.

Block is nice because it's free. As a weapon property, there is no trade-off involved in blocking. There are two advantages to OP's suggestion. One is that you are no longer losing out on stats by gaining parry. Second, it gives us transparency on how much of a defensive advantage PLD gains from block, and makes it easier to normalize PLD's 1h block and parry rates against the 2h parry rates of the other two tanks.

If you want there to be a dedicated but meaningful "tank stat", then it will either end up being a constraint (meet this minimum value in order to avoid dying), or something you try to maximize when actively tanking (i.e. a counterattack stat). I personally see the latter as more interesting, but it also runs the risk of further forcing tanks to specialize into MT and OT roles based on their gear choices, which I'm not so keen on.