Its simple. Riot made this game a mess. There is a situation, that dmg output is so high, that even buying full armor/mr/hp you are squishy.
Other thing is that being full dmg build you are so squishy, that you die when some1 will fart on you.
Third thing is that there are champions that build full tank still have a lot of dps due to their mechanic (Cho, Darius etc)
4th thing is that there are some stupid champion that build full dmg they are still tanky (J4, Malph, Cho etc)
I really miss time when tanks were just tanky and CC-dealers, when even feed ADC was still squishy but he dealt dmg.
Time when early game war for offtanks, midgame was for mages and late for ADC.
THAT WAS BALANCED. NOT LIKE NOW, THIS SHITTY MESS!!!