Here's the issues with armor and AC in general:
-AC is only a small measure of a character's defense. You also have to look at things like saving throws, spell resistance, hit points, various special abilities like evasion and mettle, etc so it's not usually wise to invest a disproportional amount of resources into significantly raising your AC.
-AC is easily replaceable with spells/abilites like mirror image, blink, displacement, wings of cover, wind wall, invisibility, etc. Plus there's plenty of AC boosting spells like alter self, polymorph, haste, barkskin, shield of faith, shield, mage armor, magic vestment, etc and most of them are fairly low level (3rd level or less) so it won't be a tremendous use of resources at mid-to-high levels.
-Generally speaking you're better off investing in several different smaller AC types (armor, shield, natural, dodge, deflection, sacred/profane etc) than focusing your resources on one big AC boosting item.
-After a certain point AC simply doesn't really manner against some combat monsters unless you get your AC very high. But like I said with my first investing a significant amount of resources in AC can easily leave you more vunerable in other areas of defense.
When you add up all of these factors it's very easy to see why armor isn't that important other than providing various armor enchantments to characters.