Frankly, the most distinguishing feature for a tank was in my view the plate armor mastery stat, and not con.
But I think that might be as intended, to give more credence to the healing classes. I still remember quite clearly that I was very self sustained as a knight in Ragnarok Online, because not only could I wear items with higher armor mitigation, the vit def also reduced attacks further, and the swordy passive health regen stat made wait times inbetween fights much smaller. Factor to that the increased potency of potions for when I DID get in a pinch, and the increased carry capacity for said potions due to strength, and I really seldomly had to party up while grinding to higher levels.
That said, the reason con gives no armor might be that unlike Ragnarok Online, equipment is easier to upgrade, there are more gems than there had been cards, and items don’t have a relatively static DEF rating. (After all, you rarely got items that went into the 10 DEF range for armor, even with cards, so you needed something for tanky classes to scale with the levels, whereas items in Tree of Savior are now scaling with levels more and giving better armor rating every few levels)