Try being a freaking melee with 0 or 1 gapcloser on a long cd. This is why every melee damage dealer builds to 1-shot adc's even if it leaves them overly squishy. Because if they don't 1-shot the adc or midlaner, they'll get cc'd and the adc becomes literally uncatchable again unless you can survive 8+ sec of focused fire from the adc and their 4 teammates (you can't... not even close). So it's nuke or be nuked. And yet how is it fair for adc's if they aren't allowed to build any defensive items until MAYBE at 6th item and get 1-shot because the enemy team's jax or renekton or whatever has flash and q/e (respectively) up? Just because they're on the same screen, they die w/o having a shot to fight back? That's not fair either.
This is why... burst and adc dps needs to be lowered. Adc's need to be free to invest in say... 3rd item ga or maw if they want to. Sure they'll lose dps, but they also make it harder for the other team to nuke them while the enemy adc is more easily punished for being caught by a burst mage or toplane bruiser/fighter. And while adc's should be allowed to have a chance at not getting 1-shot, the flip side is that the whole movespeed bonuses they get in combat via stuff like crit items needs to be removed and possibly their base movespeed lowered a bit. Because it's crept up far too close to the base movespeed of melees and drastically exceeds that of melee's in combat thanks to stuff like crit. Lowering this to where they can be caught in a reasonable amount of time (even while adc is kiting) such that melee's have a chance to re-acquire (after being peeled/cc'd for a sec or so) and kill their target makes it fair for melees. This is the counterpart to giving adc's freedom to better invest in durability earlier into the game and making it harder to 1-shot them. It has to be BOTH to be fair. More durability, less mobility. More counterplay, less 1-shot or you lose.