Leaving aside the other things because, being honest, I just don't care enough to keep arguing on them...
Because, with the exception of the (non-caster) Fighter and Rogue, you have ready-to-hand explanations for the survival. Paladin/Cleric and (as much as it bothers me) Druid/Ranger? Divine power preserving your life. Wizard/Sorcerer/Bard/Warlock/EK/AT/Artificer? You have literal reality hax. Barbarian? Your anger is literally so incandescent, you brush off burns and lightning.
Why is the Fighter or Rogue surviving these things? They demonstrably ARE surviving them, we know this because they're reaching high level. But there's no explanation for it--other than that they're Just That Awesome. For Fighters, it's preternatural durability or might. For Rogues, it's incredible agility or sleight of hand.
Give them other Extraordinary abilities in the same vein, because they're already Extraordinary simply by being high level.