In many ways, this is a return to form for D&D. If you look at, say, the Rules Cyclopedia, and other classic editions of D&D, Armor Class is presented as a matrix. Modifiers were relatively small and few. Fighters got an increase in their chance to-hit most levels (each level, when you get to AD&D), and by high level could hit the AC of most monsters. Only when you got into things like demons, angels, and very old dragons did you see AC comparable to plate mail +3 and a +1 or +2 shield.
Contrast that to D&D 3e, where a Dire Bear, being CR 7, had a "natural armor" bonus of +7, the same bonus granted by half-plate armor. An adult red dragon has an AC of 29, which is 12 points higher than average person wearing the best-nonmagical armor. It's AC reflects a value twice that of even magical plate armor. This is a cinch for the attacking fighter, who might have a to-hit bonus of +30 or more, but is much harder for the rogue or monk, whose base attack bonus doesn't grow as fast. If the wizard has to resort to a crossbow, they essentially can't hit.
So in 5e, pretty much every "tough" creature, until you hit Challenge ratings higher than 20, has an AC between 12 and 21. Likewise, the proficiency bonus for PCs goes from +2 to +6. Even someone with a flat +2 proficiency bonus can hit an AC of 21 (they need a 19), while someone with +6 bonus and +5 strength and a +3 weapon has a +14, and might still miss (they need a 5). So the two scales are both pretty "flat" already, but it also means that a fighter and a wizard have the same basic "to-hit." So in 5e, you become more powerful through an increasing emphasis, like Strength and special Fighter abilities for melee, or Intelligence for wizards. But the main differentiation between low and high level characters, and monsters, is hit points and raw damage output.
Arguably, while 3e got too unbalanced, 5e might be a little too flat. But the game is playable, balance is pretty easy to enforce, and almost all encounters involve at least some risk or danger. It's not as true in 5e as it was in classic D&D that a group of mid level characters could gang up on a powerful dragon, but it's more true in 5e than it was in 3e. This is especially important for set pieces, where you want a boss (more powerful than the PCs individually) and minions. The boss should be a higher grade of enemy than they usually face, but shouldn't be mathematically overwhelming; the minions are trivial, but shouldn't just be a fancier way of indicating "difficult terrain".
5e mostly addressed "bounded accuracy" by making hit points and damage output fairly linear, keeping base bonuses fairly flat and allowing specialized gear and traits to make the difference, and capping the game pretty firmly at 20th level PCs, with Challenge 21 to 30 representing truly extraordinary foes that would never be trivialized.
It doesn't entirely "work" but it keeps the game together, especially in those crucial levels from about 6th to 12th, when you want a wide variety of both mundane and monstrous foes. When I say it doesn't entirely work, I mean there are quirks, like how NPC warriors tend to have big piles of hit points for the beating they are about to take, very different than the traits of PCs.
