That's true and for many groups it works admirably.
Others, however, view D&D more like a game. A game in which you can do better or worse.
A +3 shield wouldn't break anything if the bard or warlock in my campaigns got in, since their AC was low to begin with.
But a +3 shield in the hands of the AC 21 Paladin... well, that would borderline break the game, since his AC would be a whopping five steps higher than anyone else.
Suddenly I'm faced with having to choose between stepping up my game or not.
The problem is: if I add monsters that can handle AC 24, that would obliterate the AC 15 members of the party. But if I don't add such monsters, the Paladin has effectively become invulnerable.
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Your point of view is a decent one to have, Adam.
But only if you are aware it's not the full story.