Everything does too much damage for two reasons.
masteries and runes. and gold generation
riot increased ambient and tower gold few seasons ago. but it slowly creeped up with tower gold increasing. first blood etc. and now the shutdown gold.
and nerfing masteries won't work. even if they cut all by half. your in a team fight. poppy hits you with grasp. shen hits you with that AOE damage. you get hit by a scorch here. scorch there
peripheral damage adds up to your healthbar. that damage didn't exist years ago. now its enough to alone kill you if you add it all up.
you can look at pro games from 3-4 years ago at same time and same cs and you will see that now they will have at least half an item more. if not a whole item. if your fed. you will probably reach full build before 30
= old item : Only gives extra AoE magic damage with your next AA but needs charges
= Gives extra damage, lethality, and ward-cleansing passive
= old item : Gives AoE damage around you as a DoT
= New item : Gives movement speed with charges, and a slow on your next basic attack + extra damage
: Old item (old version) : only gave life steal with a stacking mechanic that granted AD and lifesteal
: New item : changes 30% of the damage taken as a DoT and also gives spellvamp
are the most notable ones, they don't even need 40% of their kits, it's just training wheels to forgive mistakes.