damage has too many scalings attached to it whereas defense doesnt.
physical: attack speed, AD, crit chance, crit damage, armor pen (flat,reduction, %), scales on abilities, cdr, % dmg items which also scale with pen + attack speed. also life steal converts damage into sustain + tankiness.
magic: cdr, ability power, magic pen (flat, %), scales on abilties, % dmg items which also scale with pen + cdr.
damager dealers get 8-10 multiplying factors to increase their damage potential + can convert that into sustain/tankiness.
meanwhile all tanks have is MR, Armor, AA reductions and hp. very few tanks have abilities that scale with defense. hp regen as a base stat is useless unless you get warmogs. but if you get warmogs, u sacrifice resistances making you vulnerable to 1/2 of the enemy damage. other damage reduction scaling factors are too minimal (masteries blocking 2 dmg from champs?)
tanks get 3-4 multiplying factors to increase tanking potential. only 3 can be combined to multiply (resist + hp + special reduction).
solution: make tanks have abilities that scale with resistances + hp on top of ad/ap. add more multiplying factors for tanks. buff tabis/randuins special passive.