This way people who build glass cannon can die in a few hits, and people who build tank don't do any damage.
glass-cannons already evaporate near instantly.
also - in typical RPG the tankiest units will often only be twice, maybe 3 times as tanky as the glass cannons, and damage heavy rolls will only do about double the damage of the lowest DPS rolls. Compare a Demon Hunter, Barbarian, Wizard, and Monk from Diablo and you're not looking at a valley between clear time differences.
In LoL? AD, critical strikes, and AS allow marksman to deal around 8 times there starting damage. Grab a tank like Malphite or Taric and you can easily be 10 times harder to kill than a squishy. Ever run into 6 Warmog's Mundo? Dude can, for all intenets and purposes, go anywhere he pleases.
If you compare a low-damage champion (high CC/utility) like Taric and compare that to a high damage (no cc/utility) champion like Yi - you'll notice a massive valley between tanky and damage within itemization as-is. The AoS/MoBA itemization system already allows for much greater differences between character attributes than most other games even come close to.
Exaggerating that further seems like it would cause more problems than it has benefits. Is giving a squishy only 0.5 seconds to live rather than 1 helping the game's health any?
Also - defense is not counter-balanced with damage in LoL.
CC and utility are counter-balanced with damage.
Defense is largely treated as one of many defensive mechanics a champion can use to survive long enough to do their job - just like range and mobility.
Roughly half the fighters in LoL fall under a tanky DPS catagory where they are actually major damage rolls capable of matching (or exceeding) the total damage dealt of marksman, assassins, and mages that go glass-cannon while building heavy defense or occasionally hybridized offense/defense.
To a lesser extent - there's also a solid number of champions that need a splash of defense due to not having the right mobility/range to get their intended job done.
Splitting defense and offense apart can really hurt such champions.