Oh, and AC represents how much protection you have on you to avoid taking any damage at all. It is the sum total of all your defenses - armor, ability to move out of the way of an attack, deflect an attack, etc.
This is why a rogue can actually have higher AC than a fighter. A fighter wears chainmail with AC 16 but no Dex bonus. Rogue wears Studded Leather + 2 with Dex +4, so AC 18 (14 for Studded Leather +2 and +4 for Dex). The rogue's armor is easier to penetrate, but they are so fast that they can avoid getting hit to begin with.
But if a fighter avoids damage wearing their chainmail, it is assumed that their armor did the work, for the most part. They still moved to avoid the hit, but the weapon likely connected in some fashion. The armor did the brunt of the work, though, to keep them from sustaining cuts and bruises.
An equipped shield provides +2 AC bonus because it can be moved about in any direction to deflect enemy attacks coming at a PC. The enemy could be behind, in front, to the right, to the left, it doesn't matter. The bonus is because they have something in hand to help slap the attack away. If it's worn, it does no good because it is, as someone mentioned above, bulky and if anything slows down and encumbers a PCs ability to avoid getting hit to begin with. It might cover 30% of your back or even 50%, but when an enemy is coming at you, how many people turn their backs to them to be awarded the bonus of a shield on the back? Most try to face an incoming opponent and then dodge away. OR... they pull their shield out and try to use it to then deflect. Few will actually duck down so their enemy strikes their shield on their back.
