Mechs are not built to be invincible silly.
The Atlas is.
"a 'Mech as powerful as possible, as impenetrable as possible, and as ugly and foreboding as conceivable, so that fear itself will be our ally."
unfortunately they threw speed out the window to achieve that
Good point, but to be honest this thread would probably last longer and be more enjoyable if it was a general mecha megathread.