Look at the bigger picture.
It's true that as a zerker, you don't get any ascendancy-specific bonuses, and a defensive malus (10% dmg taken).
But what do you get a whole package?
A ranger start gives you easy access to dodge, a bunch of really inefficient life nodes, and more of evasion that you don't really need. You get none of actual damage reduction.
A marauder start gives you a shitton of life, a shitton of % armour that stacks REAL good with armour-based gear AND with Molten Shell, and easy access to extra endurance charges. If you go sword-and-board, you can get a good deal of ez block with marauder. ALSO, soul of steel.
Both characters can utilise fortify (but marauder has a relatively easier access to a nice fortify boosting node), easy blind via passives or abyssal jewels (but for evasion-based builds blind hits diminishing returns HARD, while for armour-based builds it's a straight 0->50% attack evasion boost), flesh and stone (gives ez blind, but more importantly - a dmg reduction for anything not close by: read as "OFFSCREEN DAMAGE REDUCTION").
If you add that all up, then sure, you can get 92%+ attack avoidance with eva-stacking Raider (with blind, dodge and a bit of block factored in), 40% spell damage avoidance, with fortify and maybe a few other small damage reduction bits. You get fuck all life, though.
As a zerker, you get a souped-up fortify, tons of armour AND molten shell, easy blind (so 50% base attack avoidance with near-zero investment), soul of steel, and also you can get extra endurance charges and shield block if you feel like it.
But in reality, it all pales in comparison to the damage disparity - zerker just deals SO much more damage. And the best defense is still a good offense. While raider keeps hacking at a boss hoping that RNG-based avoidance will save them, a zerker simply goes in, tanks a few hits because the build can afford it, and then kills the boss so much faster than zerker.