Why do ADCs need to be so squishy?

Like the tittle suggest,we all know how squishy they are and a lot of people find more and more frustrating to play adc role cause before you get your build you are a walking caster minion and even after you get your build if your team doesnt peel for you you most likely get oneshotted by literally everyone,so my question is,why does every other role get some items that have both AD/AP and HP except for ADCs(not counting very few adcs that can build black cleaver or trinity force that give them some hp),maybe let me clarify the question,why isnt there some item that is viable on adcs that also gives health stats,why do ap champs get things like rod of ages,protobelt,rylai,liandrys etc. that work perfectly well with their kits,while adcs are forced for most part to build full ad? Dont get me wrong,im not suggesting to make adcs into unkillable tanks that also do tons of dmg,but just maybe they need that litttle extra 250-300 hp,cause trinity and black cleaver dont work on most adcs