When Dave Arneson set out to create the combat system that would become a pillar of Dungeons & Dragons, he did not aim to create a realistic simulation. In a 2004 interview, he describes the system’s genesis from Gary Gygax’s Chainmail rules.
Combat in Chainmail is simply rolling two six-sided dice, and you either defeated the monster and killed it…or it killed you. It didn’t take too long for players to get attached to their characters, and they wanted something detailed which Chainmail didn’t have. The initial Chainmail rules was a matrix. That was okay for a few different kinds of units, but by the second weekend we already had 20 or 30 different monsters, and the matrix was starting to fill up the loft.
I adopted the rules I’d done earlier for a Civil War game called Ironclads that had hit points and armor class. It meant that players had a chance to live longer and do more. They didn’t care that they had hit points to keep track of because they were just keeping track of little detailed records for their character and not trying to do it for an entire army. They didn’t care if they could kill a monster in one blow, but they didn’t want the monster to kill them in one blow.
So the D&D rules for hit points and armor class stem from rules for ironclad ships trading cannon blasts, hardly the basis for an accurate simulation of hand-to-hand battles.
Soon after I began playing D&D, the unrealistic combat rules began to gnaw at me. In the real world, armor reduces the damage from blows rather than making you harder to hit. Shouldn’t it work the same way in the game? And how could a fighter, no matter how heroic, survive a dozen arrow hits, each dealing enough damage to kill an ordinary man? In reality, a skilled fighter would stand a better chance of evading blows, but no better chance of surviving a single hit.
Quest for realism
In the decade after D&D’s introduction, a mania for creating realistic alternatives to D&D dominated the hobby. Every D&D player who ever wielded a sword in the Society of Creative Anachronism cooked up a more realistic alternative to the D&D combat system. Runequest (1978) stands as the greatest early success. Characters’ hit points remained constant, but they became more able to dodge and block blows. Hit locations transformed characters from blobs of hit points into flesh and bone. Armor reduced damage by deflecting and cushioning blows. 
If you enjoyed the AD&D Weapon Armor Class Adjustment table, but felt it needed to go much, much further, the Rolemaster Arm’s Law (1980) system offered more than 30 tables matching weapons versus armor.
In this era, everyone formulated a critical hit table, because nothing adds fun to a system like skewered eyes, fountaining stumps, and sucking chest wounds. (Follow this blog for my upcoming list of supposedly fun, but not fun, things we did in the early days of role playing.)
I sought realism as much as anyone, first with Runequest, and then with GURPS. I quickly learned that making combat more realistically deadly made D&D-style, combat-intensive play impractical. Forget dungeon crawls; even skilled characters would eventually perish to a lucky blow. As I described in Melee, Wizard, and learning to love the battle map, early D&D combat lacked excitement anyway, so I hardly missed all the fights.
But I would come to realize that my dismissal of the D&D combat system was completely wrong.