It doesn't. Even the CR 17 Death Knight is only +11 to attack, so needs a 19+ rendering it free XP. Most monsters in the game need a 20. Rendering monsters unable to present a challenge to the party due to being unable to hit means bounded accuracy is completely busted. There are plenty of games out there with bounded accuracy, this is definitely not one of them.
Bounded accuracy just means putting an upper and lower limit on bonuses that are within parameters that make sense. In this case it is impossible to get an AC so high that the best monster in the book needs a 20+ to hit you. That means that accuracy is bounded. This is in contrast to situations where it was possible to get ACs of 50 or 60 in 3.5e and make the best monster in the game need 30s+ to hit you.
The numbers are bounded to the d20. If you follow the guidelines, monsters of around your level should never hit on a 2 and never hit only on a natural 20(on the members of the party that are meant to be in melee combat).
That's what the math was designed to do.
Plus, a Death Knight doesn't need to hit with its sword to kill a party. It can do a fireball that will hurt a party really bad regardless of their AC.
AC 28 is a 20 barbarian with nothing but a nonmagic shield and codpiece of modesty buffed with haste and shield of faith.
AC 28 is a fighter 2 wizard 18 with nonmagic plate and shield buffed with haste and shielding himself with his free shield spell.
AC 28 is not even HAVING a magic item. Give that barbarian the helm from HotDQ and a +3 shield and a ring of protection and he's at 37 and even CR 24 monsters need a 20 to hit him and CR 30 needs 18. Give the wizard +3 plate and shield and a ring +1 he's a 35.
Above 20 isn't awesome. 23 is your generic 2nd level (or whatever level they can afford it) paladin in nonmagic plate and shield with shield of faith up.
You really need to put these in context. Plate mail is the best armor in existence. It is the stuff the richest people in the world wear to protect themselves and is the pinnacle of technology. With that and a shield most people are nearly invincible when facing mundane threats. Most commoners/kobolds/goblins are going to miss 80-85% of the time. You can face down a number of foes in that armor and be fairly certain you'll come out alive.
20 is pretty awesome. Anything above that requires significant resources most people don't have like magic. If the PCs do have it...well, congrats, that means they are SUPER powerful.
The problem is that a lot of players have been playing 3.5e for so long that their idea of what a D&D world should work like has been calibrated by that game. They expect guards to be equipped with +2 weapons because the city has hundreds of them laying around. They expect every enemy they come across to have shield of faith, haste, shield, or any number of other buffs up at any time.
5e creates a fundamentally different fantasy world than 3.5e used to(which is true of 2e, and 4e as well). Each edition the expectations have changed slightly.
I imagine the world of 5e D&D is one in which the use of a spell like Shield of Faith to protect someone is considered truly magical and those who can do so are considered to be very rare and powerful. Magic items are treated with awe and reverence because of how few of them there are out there.
If you play that game, the assumptions described make perfect sense. So far, from my experience with Adventurer's League it seems to be the expectation that most PCs will get about one magic item every 4 levels or so. Or about 5 magic items by the time they are level 20. I would expect the average PC to have either a magic weapon OR armor but not both. I would also expect that by 20th level most PCs should still have +1 items with the rare PC having a +2...with +3 items something that 1 PC might get by the time their are level 20...but likely only once every 2 or 3 full campaigns.
