Armour and Penetration - build more armour?

So the general guideline within the game is that AP penetrates their calibre / 14.3.

I'm going to guess that is consistent with actual physics / actual history?

Why didn't they just build thicker armours? Yamato guns penetrate up to 32mm of bow armour, so if you built a ship with 50mm of bow armour all around you'd be basically impervious to any and all rounds since things would just bounce off?

Some citadels are heavily protected with >300mm of armour. What if we tripled it and protected it with 900mm? it'd be basically an impregnable fortress on the high seas, right?