yes, building resistances feels pretty bad right now, especially later on once your opponent have %pen.
Going for maxhealth makes you more tanky, though not by that much.
Tankiness just gets heavily outscaled by damage. Early to mid with 1-3 items, tankyness is great and some tanks seem unkillable, but as soon as the carries get 4+ items, you start to melt away no matter what you built, unless you have some sick defensive mechanisms like an alistar ult, or you simply cc and kill the damage source.
As for the 3 champs you mentioned:
They have high basevalues, and the LW items are based on bonus armor, Poppy has a small hitbox and moves rather fast with W and E, makeing it hard to really focus her, especially with skillshots, while sion eats all the skillshots no matter if they were aimed at him or not, and her W prevents anyone with a gapcloser from getting on her, no gragas E+W fpr peel, no Xin dashing on her and following up with Q, no LB W and so on, so few but her primary target will actuallyhit her, and the prime target usually is stunned for 2 sec aswelll...
The main damage still comes from ADC-AAs, and Jax can didge them for 2 sec and usually stun them for another sec afterwards, so he effectivly takes next to no damage for the first 3 sec, and just as oppy his Q mobillity and small hit box make is unlikely that he gets hit by stray skillshots and it's harder to focus him.
And often enough he kills the adc rather fast, so the main damage source uis taken out befor it can do alot.
Garen has the 30% damage reduction from W, which unlike resistances isn't affected by pen and thus actually makes him tanky. W + base armor reduce physical damage by +60% for him already, and non of that is affected by LW, so he actually gets rather tanky and mostly needs hp tp back up the damage-reduction.