In D&D objects have hitpoints values, but when I equip a suit of full platemail it doesn't grant me protection in the form of its own hitpoints before any actual damage is applied to me. Have any D&D designers said why armor is designed this way?
I'm not sure this answer is exactly what you were expecting, but the question has got none yet. So I'll take a risk to make use of your exact wording "any D&D" (though you may have meant "any designers").
From AD&D 2E DMG:
For the most part, specific damage isn't applied to equipment under the AD&D rules. This doesn't mean that equipment is never damaged or broken. Instead, it is assumed that whatever normal wear and tear an item may suffer (such as dents in a suit of plate mail) are repaired during moments (or days, or months) of inactivity.
The fighter spends time in camp sharpening his weapons, patching the rips in his chain mail, and hammering out the dents in his breastplate. The thief repairs the padding that muffles the clinks of his metal buckles. The mage sews patches onto his clothes. All characters have ample time to make repairs. It's not very interesting to role-play, so it is assumed all characters maintain their equipment.
Thus at least designers of the previous edition explained it explicitly. They didn't think it was interesting. The concept of damaging objects has been taken to the 3e with only few changes (it was expanded with sunder combat action and object hardnes but the core concept remained the same). So we can assume that designers of D&D 3e either shared this view or just have taken the concept as is.