Just one case of the disparity between damage and defensive items

Health 1200 + Magic resist 990 = 2190 Effective combat stats Health 1200 + Magic resist 990+ Health regeneration per 5 seconds 720 + Cooldown reduction 266= 3176 Stats without passive

Health 800 + Ability Power 1740 = 2540 Stats without passive Health 800 + Ability Power 1740 + Magic penetration 270 = 2810. Effective combat stats without passive.

You can do this with nearly every ap and ad item and the damage item will come out on top nearly every time unless its based on being a support item with weak stats but actives and passives or maybe . Morellos is more combat effective building ap helps you get gold through farming and kills while building magic resist and hp doesnt until you build or you one of the few champions that have Damage scaling off of your own max hp. then it CAN do that but usually only for one ability on a long cooldown. Magic resist like armor is less effective for every 100 points you buy unlike ap. 20 ap will give you the same scaling as it does after 100 ap as it does after 300 ap. while 25 magic resist will give you less damage reduction versus magic damage every 100 magic resist you build making the first 100 points of Magic resist most valuable and most people reach 70-80 magic resist just through level scaling same for armor. tank items are loaded up with useless in fight stats that make up their gold value same with bruiser items while health isnt useless its less useful than damage without being at least base tanky enough and having enough lifesteal or allied healing (that all gets pumped onto adcs and bruisers dont get any in this meta) while every bit of the damage you build will always have the same output.

I normally dont ever go this much into detail about gold because I know that the items are more infavor for damage than they are defense and because its tireing. This is why all the tank items are cheaper but at tanks Have to build defense versus physical damage magic damage and true damage but arent allowed to tank more than 4 seconds because they easily have more than 4k gold value disparity in their just from getting the max amount of stats not even caring how thing react with each other and Bruiser have it worse because they have to build defense versus Magic damage physical damage true damage and have to build their own damage.

Edit explination of how the returns are diminishing

In reality even though will always give 25 magic resist, the damage reduction versus magic damage diminishes thus every point above 100 magic resist. If the first 100 armor/ magic resist because they negate 50% damage of their type this is 100% efficiency they are worth 2000 gold and 1800 gold respectively then the second 100 armor/ magic resist at 200 armor/ magic resist are worth 34% of the original amount because they then negate 17% additional damage of their type with a worth of 740 gold and 666 gold respectively and at the 3rd 100 armor/ magic resist at 300 armor/ magic resist are worth 16% of the original amount because they then negate 8% additional damage of their type with a worth of 160 gold and 144 gold respectively.