so if its not ok for bruisers to get damage by building defense. why is it ok for carries to get tanky by building offense?

adc have the strongest possible defensive stat on the strongest offensive item right now. even tanks dont get access to % damage reduction items.
and the % damage reduction of course synergizes with the bloodthirster shield.
by the way the reason atmas was officially removed was because it was a noob trap after all its nerfs, and its stat combination with crit in there which made it nieche for only 2 or 3 champions.
the reason it was nerfed was because of the midgame powerspike it caused due to its scaling off base health. it wasnt just good for fighters. stat wise an atmas rush was already good for carries who trade with other physicald damage dealers like other adc.
lategame there was never anything wrong with atmas. but the midgame powerspike in combination with mallet or the old warmorgs (which granted 1350 hp fully stacked) was why the item got nerfed.
titanic hydra also works diffently in that it doesnt grant a direct ad bonus, but on hit damage which doesnt increase ability damage or is subject to critical strikes.
if youre stacking a lot of health to get the on hit bonus up, youre not stacking a lot of attack speed to synergize with it. while atmas + health stacking did increase damage on ad scaling abilities.
and finally, anti health items and kits didnt exist back then or werent nearly as strong. giant slayer, bork, liandries and all those champions with % health damage all were introduced after the atmorgs phase. if they existed back then, the game might have looked different.