Block and parry do very little to reduce HP damage, but HP isn't by itself health. It's just a numerical representation of how durable you are until you receive a fatal (or knockout) blow.

Nobody is going to be alright after taking a direct hit from a missile, but characters in the game can. Is everyone secretly a superman? Is it because it's a game? Or is it because the tanks are already doing something to survive those kind of hits that other classes can't (deflection, blocking, and parry)?