My thought is, I've got full plate. If you attack me with a sword, you've got to aim in some very precise spots to hurt me, so that's why my AC is high. If you attack with a pick, if you can hit one of those precise spots, I'll be really hurt, but even if you miss, your pick might go through a plate and still hurt me. It just won't be as much, because the metal absorbed some of the damage.
The logic works the same way with other armors. You can stab a guy in chainmail in his armpit or throat, or you can wallop him with a mace in his chest, and still hurt him a bit.
Bear in mind, my group liked Conan d20's combat rules, where you could either try to bash through armor, or hit a weak spot, so a little fiddly-ness is not big problem if it gives us more combat options.