D&D is not realistic.
D&D combined Armor and Evasion in calls it AC. But in reality different Armor types are prone to different weapons.
Means a dragon with AC 19 is not hard to miss but his scales are hard so you need a 19 to make damage. But a Axe is different than a sword and needs maybe only a 17 etc. ...
D&D is very streamlined in this way because AC is a fundamental Pillar of Danger Level and Enemy Strength.
