Defenders need high defenses, yes. But the idea isn't to become unhittable. Unlike in a video game, there is nothing keeping a monster from attacking your friend instead of you. The purpose of the defender's mark (or defender aura) is to make it so the defender is a more enticing target than the squishy strikers, leaders and controllers.
If a defender's defenses are too high, enemies will quickly realize this and ignore the defender, even when marked. The defender's punishment mechanic will help somewhat, but ... here. Maybe it's better if I show you this way:
Say you have a level 22 strength-based defender with a 40 AC (10 +12 armor +5 enhancement +2 shield +11 half-level) adjacent to a creature. Also adjacent is a dragon sorcerer with a 37 AC (10 +3 armor +5 enhancement +8 strength +11 half-level). The sorcerer took Leather armor proficiency to help his defenses somewhat.
The defender has multiclassed into Sorcerer, and has taken the utility power swap feat to get Platinum Scales. He is beset by an efreet fireblade (a level 22 monster). He is attacked, and triggers Platinum Scales. Because he has a +8 strength modifier, his AC is now 48.
The next turn, the Efreet, having already used its Whirling Firesteel Strike (which didn't recharge this turn), uses its Scimitar attack, +27 vs. AC. It rolls a 19 on its attack.
19 + 27 = 46, which is below the AC of the defender. In fact, the efreet would have to roll a 21 on a d20 in order to hit the defender, which is impossible. Thus, the efreet is not going to bother attacking the defender (since it literally cannot hit it) and go focus on other members of the party. Yes, it will take a -2 penalty to any attacks if marked, but that is preferable to not being able to hit at all.