Why is it always ad items that get broken patch after patch

Does that mean we have all champions gain MR per level and have it grow much faster than currently? Because all champs either have 38.5 MR or 53.4 MR at level 18 as their base magic resist before items. Meanwhile, armor on champs ranges from 65.5 to 104.5 base armor meaning a champion with 1000 health (Just a quick number) has either 1385 or 1534 effective hitpoints against a mage and 1655 to 2045 effective hitpoints against an attack damage dealer.