Everyone starts squishy. Typically start one damage item expecting to end laning phase and knock people down. Then people look at the (EXAMPLE!) Triforce Fizz and ask, when they have no bonus defenses "why can that one shot me." That's because the baseline for life expectancy is pretty low in this game and Triforce is nearly 3733g of damage with a little bit of it being health and utility. Only support characters really aren't lethal on utterly defenseless mid level targets with near 4k gold in damage.
From there a mage or adcarry (or bruiser!) either build more damage to deal with the tank walls. Or they bulk themselves up which is almost always wise. You kinda need to. Other than fighting full defense you don't really NEED more damage. Instead it becomes wiser to raise your life expectancy versus your greatest threats by 50% rather than buy an item that does like 10% more damage. Mainly talking about AP itemization 
and in the past 
Your total character strength with an "offense rating of 2" and "defense rating of 2" multiplies to 4 strength as opposed to a 3x1 loudout. In the case of mages their offense specialty items barely promise 10% more damage at best so they frequently build "conservative" stuff. A few mages have overwhelming damage baked into their kits like Cassio they build like ranged AP bruisers. So gross.
There was once a time in league bruisers were consistently the best duelists with their balanced offense/ defense loudout. They still tend to be but require fervor of battle rapidly stacking for them lots of ad as a crutch.
ADC's crit items were recently buffed above the Bork bar that was recently pushed up and now ADCs freely and comfortably rush their end game build. Dunno what to say about that. Positioned properly they have most of their team to cover for them. What happens is when they pick up extra defenses (and also have their support defend especially them) they get more mileage out of every ounce of hp/resist an item gives them than someone who is expected to dive. So why is it special ranged concession have to be made for like duskblade but not hexdrinker for example? Might be because Riot more aggressively tunes damage stats but not defense stats. (same for characters)
After some years the patterns are very predictable
How would I change this? Instead of mandatory defense buys and specs I would raise hp per level across the board. All characters (I think). But lower resists on carry defense hybrid items to be about the same effective life (or whatever is not too much), but emphasize their one off or needs to be properly timed active properties if needed lowering passive durability particularly for ranged mages and carries. Higher hp levels in game would give more headroom for burst items to be good but not overbearing. It would divert carries to building more damage and less defense to meet damage demands + they'd have slightly more comfort to. And I think on the side tanks would have more hp to "scale" their resistances with. IMO tanks need a better late game in particular.
The drawback might be the possible feeling of blandness from if hybrid offense/defense items are too mute to appreciate.
I'm not sure where this puts bruisers, but to me the signs seem to say bruiser champions are underpowered but most stuff in the system that supports those champions are overpowered (and tend to get poached). In my opinion defensive stats regardless of class are overpowered in general because of the way the game is designed atm
