Balance comes from the whole rather than a part.
Those games with atk/def calculation ends up having to scale up exponentially because both HP AND DEFENSE goes up. Eventually it’ll get to the point where the lower defense cloth/leather class just get one-shot:
For example, say a tank have 1500 HP, 75% damage reduction, if a boss is to kill said rank in 3 hits then it’d have to hit for at least 500 post-reduction, i.e. 2000 before. But by logic of weaker armor + range class, the range player have only 1000 HP + 40% reduction…so he dies in one hit because the gear doesn’t offer enough defense.
And buffing the range’s defense/nerfing the boss’s damage aren’t possible because:
buff range’s defense - this messes with PvP
nerf boss’s damage - this makes tanking and healing even easier, and the fight loses its tension because unless the healer fall asleep the tank, now lasting even longer due to lower boss damage, won’t die.
Games like Starcraft, DnD, etc have also found balance even without using % reduction.
Or heck, look at XCOM if you want a more recent example.