I play Jayce and everytime I go top lane against champs like 
etc., they take a sliver of damage from all my abilities, and then one of their abilities does about 25% my health. Wtf is this? Am I supposed to build all lethality runes or something? Because thats not gonna do anything beens theyve been nerfed to shit now.
-Gragas is an AP fighter. He can get away with building tanky early, but he needs damage in order to not fall off later on.
-Swain is a mage.
-Malphite is a tank that hard counters most AD champions until they pick up penetration
-Chogath is an AP juggernaut.
if you can't lane against these champs as Jayce, I would suggest practicing, because he's a champion that improves as you improve.
those aren't tanks
those are fighters who happen to be good at "tanking" damage but they aren't actually tanks.
Sure the label says tank, the label that's there for noobs that is. But they aren't. Some will disagree with me, but at the end of the day its not a coincidence that you are left confused as "tanks" beat your face in
Tanks are usually a bit strong early.
tl;dr: git gud
Because they're all AP and use mana, so their spells hurt extra hard. Meanwhile they all have natural defenses/sustain to mitigate Jayce's pewpew damage
You should be able to win any of those lanes tbh.
Build an early hexdrinker then BC. then a lifesteal item. So basically defensive damage into sustain.
Could also start triple dorans if you're desperate.
