AC isn't meant to simulate armor as damage reduction, it's meant to simulate armor as damage avoidance. The idea behind it is that armor doesn't reduce the damage you take, it instead prevents you from taking any meaningful damage.
An example of how this would play out IRL is is you try to slash a person wearing plate armor with a sword. The armor won't reduce the damage of the sword but will instead prevent the sword from doing any damage and have it just bounce off the plate. Beating an enemy's AC represent doing actions that get around the protection of the armor such as striking in exposed areas or hitting with enough force to bypass protected areas.
Another example would be blocking an arrow with a shield. The shield won't reduce the damage an arrow does but instead you won't take damage in the first place since it would have been blocked.
If you want standard damage reduction on your character, you should look into the heavy armor mastery feat (reduces any non-magical b/p/s damage you take by 3) and the resistance system.
